Kitchen Science #6: Solar Science, 3 Illuminating Experiments to Harne

June is Britain's brightest month—long days, high sun, and the kind of light that makes even grey England feel luminous. The sun that coaxes gardens into summer is doing something far more dramatic than warming your patio: it's delivering 173,000 terawatts of energy to Earth's surface every second. That's 10,000 times the total energy used by all of human civilisation, arriving free, clean, and inexhaustible. Solar energy isn't just a future technology; it's the foundation of nearly all energy on Earth. The oil in your car, the coal in power stations, the wood in your fireplace—all are stored sunlight, captured by photosynthesis millions of years ago. Even wind and hydroelectric power are solar-driven: the sun heats air to create wind and evaporates water to create the rainfall that fills reservoirs.

This month's experiments harness June sunshine directly, exploring the science of solar energy, light, and heat. You'll build a solar oven powerful enough to melt chocolate, investigate how different colours absorb heat differently, and create a simple solar cell using everyday materials. Each experiment illuminates—literally and figuratively—the principles driving the multi-billion-pound solar energy industry and the photovoltaic cells now appearing on rooftops across Britain. The only equipment required is sunshine, household materials, and curiosity.

Experiment 1: Build a Solar Oven

What You'll Learn

Construct a working oven powered entirely by sunlight, demonstrating how solar energy can be concentrated and converted to heat—the principle behind parabolic solar collectors, concentrating solar power plants, and the solar cookers used by millions of people in sunny regions worldwide.

Equipment Needed

A pizza box (or any large cardboard box with a lid)
Aluminium foil
Cling film or clear plastic sheet (enough to cover the box opening)
Black paper or black card
Scissors
A ruler
A pen or pencil
Sticky tape
A stick or straw (to prop the lid open)
A sunny June day (ideally between 11am and 3pm)
A thermometer (optional, but fascinating)
Something to melt or warm: chocolate, marshmallows, or cheese on a cracker

Estimated cost: £0-2 (if you need to purchase foil)

Safety note: The inside of a well-made solar oven can reach 90°C on a good June day. Treat it with appropriate caution—use oven gloves when removing items. Never leave children unsupervised near a working solar oven. Do not use to cook raw meat or eggs.

Method

1. Prepare the reflector panel (10 minutes) Draw a square on the pizza box lid, leaving a 2.5cm border on three sides. Cut along three sides of this square, leaving the back edge as a hinge, so you can fold the panel upward. This flap will be your solar reflector.

Line the inside of this flap entirely with aluminium foil, as smooth and wrinkle-free as possible. Smooth, shiny foil reflects more light than crinkled foil. This is your solar concentrator—it reflects additional sunlight into the cooking chamber.

2. Create the cooking chamber (10 minutes) Open the main box. Line the inside bottom of the box with black paper or black card. Black absorbs heat radiation most efficiently—this is your heat-absorbing surface.

Line the inside walls of the box with aluminium foil, shiny side facing in. This reflects heat back towards the centre rather than letting it escape through the cardboard walls.

3. Seal the opening (5 minutes) Cut a piece of cling film or clear plastic slightly larger than the box opening (where the lid was). Tape it securely across the opening, creating an airtight transparent seal. This acts as a greenhouse—it lets sunlight in but traps warm air inside, just as a car heats up on a sunny day.

Make sure the seal is as airtight as possible; air gaps let heat escape and reduce efficiency dramatically.

4. Position and use (30-60 minutes cooking time) Place your solar oven outside in full sun, angled so the reflector flap directs maximum sunlight through the transparent cover. Prop the reflector flap at an angle of roughly 45° using your stick or straw—adjust to maximise reflected light entering the chamber.

Place your food item on the black surface inside the chamber. Check temperature with a thermometer if you have one.

Reposition the oven every 15-20 minutes to track the sun, maintaining maximum solar input. On a clear June day between noon and 2pm, internal temperatures should reach 65-90°C within 20-30 minutes.

Watch as chocolate slowly melts, marshmallows soften, or cheese begins to bubble. Your solar oven is working.

5. Experiment with optimisation (ongoing) Try different configurations: more layers of cling film for better insulation, additional foil reflectors on the sides, darker versus lighter internal surfaces. Each modification tests a principle of solar thermal design.

Expected Results

On a clear June day between 11am and 3pm, a well-made solar oven should reach internal temperatures of 65-90°C—hot enough to melt chocolate within 20 minutes, warm a small cup of water in 30-40 minutes, or melt cheese on crackers in 15-20 minutes.

Performance varies significantly with cloud cover, sun angle, and construction quality. On a hazy day, expect perhaps half the temperature. The difference between a well-sealed oven and a poorly-sealed one can be 20-30°C—illustrating how important thermal insulation is.

If you have a thermometer, plot temperature against time from initial placement. You should see a steep initial rise as the chamber heats, followed by a plateau as heat loss balances heat input. This plateau is your oven's maximum operating temperature.

Troubleshooting: If your oven barely warms, check for air leaks around the cling film seal. Ensure your reflector is actually directing sunlight in—you should see a bright reflection inside the chamber when positioned correctly. Check the sun angle—low winter sun can't reach inside the box effectively, but June's high sun should. Very humid or hazy conditions significantly reduce performance.

The Science Explained

Your solar oven works through three principles: reflection, absorption, and the greenhouse effect.

Reflection concentrates sunlight. Your aluminium foil reflector redirects light that would otherwise miss the cooking chamber into it, increasing the total solar energy captured. Professional parabolic solar concentrators use the same principle at enormous scale, focusing sunlight intensely enough to boil water and drive steam turbines.

Absorption converts light to heat. Your black paper absorbs virtually all wavelengths of visible and infrared light (black absorbs all colours rather than reflecting any). The absorbed light energy excites molecules in the black material, increasing their vibration—which is what heat is at the molecular level. Lighter colours reflect more light and absorb less, which is why dark clothing feels hotter in sunshine and why solar thermal panels are always black or very dark.

The greenhouse effect traps heat. Your cling film allows short-wavelength visible light through but is relatively opaque to the longer-wavelength infrared radiation emitted by warm objects inside. Light comes in easily; heat can't escape as efficiently. The same physics governs the glass in a car on a summer's day, the glass panels in greenhouse horticulture, and atmospheric greenhouse gases that warm our planet.

The efficiency of your solar oven depends on minimising heat loss whilst maximising heat input. Professional solar thermal systems use evacuated tubes (removing air to eliminate convection heat loss), selective coatings (which absorb sunlight but emit very little infrared), and precise optical focusing—all refinements of the same principles your cardboard oven demonstrates.

Real-World Applications

Concentrating solar power (CSP) plants generate electricity using exactly your solar oven's principles at massive scale. Curved parabolic mirrors focus sunlight onto tubes carrying heat-transfer fluid, which heats water to drive steam turbines. The Noor solar complex in Morocco covers 30 square kilometres and provides power for 1 million homes.

Solar cookers are transformative technology in the developing world. In sub-Saharan Africa, where many families cook over wood fires, solar cookers reduce fuel costs, reduce indoor air pollution (a major health problem from wood smoke), decrease deforestation, and free up hours spent collecting firewood—time typically spent by women and girls. Simple cardboard and foil solar cookers costing a few pounds change lives.

In Britain, solar thermal panels on rooftops use similar principles to heat water, reducing household energy consumption. They're distinct from photovoltaic (PV) solar panels that generate electricity—solar thermal heats fluid directly, which is simpler and more efficient for water heating specifically.

Taking It Further

Variation 1: Measure temperature every 5 minutes and plot a graph of temperature versus time. How does this change with cloud cover? Does the oven reach maximum temperature faster on a cooler day (less heat loss) or warmer day (more solar input)?

Variation 2: Build two identical ovens, one with double cling film layers and one with single. Compare maximum temperatures—does better insulation make a significant difference?

Variation 3: Test different coloured interior surfaces (black, grey, white) by lining separate sections of the oven floor with different colours and placing identical objects on each. Which melts chocolate fastest?

Related Questions to Explore:

How do solar thermal power plants store heat to generate electricity at night?
Why do concentrating solar plants work better in deserts?
What limits the maximum temperature a solar oven can reach?

Experiment 2: The Colour and Heat Experiment

What You'll Learn

Investigate how colour affects heat absorption from sunlight, discovering why engineers choose specific colours for solar collectors, buildings, and clothing—and revealing the physics of why a black car seat burns you in summer whilst a white one stays comfortable.

Equipment Needed

5-6 identical glass jars or plastic bottles
Water (enough to fill all jars equally)
Black, white, red, blue, green, and silver/aluminium foil (one sheet each)
Sticky tape
A thermometer that fits inside the jars (or an instant-read thermometer)
A sunny windowsill or outdoor space
A notebook for recording temperatures
A timer

Estimated cost: £0-5 (coloured paper available from pound shops or craft stores)

Safety note: On a hot June day, jars in direct sunlight can become very warm. Handle carefully.

Method

1. Prepare your experimental jars (15 minutes) Fill all jars with equal amounts of cold tap water—the same quantity in each is essential for a fair comparison.

Wrap each jar tightly with a different colour: one black, one white, one red, one blue, one green, and one wrapped in aluminium foil. Tape the covering securely so it sits flat against the glass with no air gaps.

Label each jar clearly.

2. Take initial temperature readings (5 minutes) Before placing jars in sunlight, measure the water temperature in each jar. They should all be the same (or very close—within 1°C). Record these baseline temperatures. This is your control measurement.

3. Place in direct sunlight (60-90 minutes) Arrange all jars in a row in direct sunlight, ensuring each receives equal light exposure—same surface area exposed to sun, no jars shading others, same distance from any reflective surfaces. A south-facing windowsill or outdoor table works well.

4. Record temperatures (Every 15 minutes) Measure water temperature in each jar every 15 minutes for 60-90 minutes. Record in a table:

Time	Black	White	Red	Blue	Green	Silver
0 min
15 min
30 min
etc.

Ensure you measure the water, not the jar surface—insert thermometer fully into the water.

5. Analyse results (10 minutes) After 90 minutes, calculate temperature rise for each jar (final temperature minus initial). Rank jars from greatest to least temperature rise. Compare with your predictions.

Expected Results

After 90 minutes in good June sunshine, you should see clear differences. The black-wrapped jar will show the greatest temperature rise—typically 8-15°C above baseline. The silver/foil-wrapped jar should show the least rise, potentially 1-3°C. White will be nearly as low as silver.

Coloured jars (red, blue, green) will fall between these extremes. The ranking generally follows from darkest to lightest, but colours matter too: dark blue absorbs more than light blue, dark green absorbs more than yellow-green. Within the visible spectrum, colour determines which wavelengths are absorbed and reflected.

The differences may seem modest—perhaps 10°C between black and silver—but scaled up over a building's roof or a car's surface, these differences become enormous. Your black-wrapped jar in June is demonstrating why solar thermal panels are black and why white roofs in hot climates can reduce air conditioning needs by 15-30%.

Troubleshooting: If differences are very small, increase exposure time or choose a sunnier location. Cloud cover significantly reduces the effect. If all jars warm equally, your coverings may have gaps—ensure each jar is completely and tightly wrapped.

The Science Explained

Colour is the visible manifestation of how a surface interacts with different wavelengths of light. White objects reflect all visible wavelengths, so no light energy is absorbed (explaining why white surfaces stay coolest). Black objects absorb all visible wavelengths, converting light energy to heat with maximum efficiency. Coloured objects absorb some wavelengths and reflect others—red objects reflect red wavelengths and absorb others; blue objects reflect blue wavelengths and absorb others.

But the story extends beyond visible light. Sunlight contains ultraviolet light (shorter wavelengths than visible) and infrared light (longer wavelengths than visible) as well as the visible spectrum. Materials can reflect or absorb these invisible wavelengths independently of their visible colour. Some materials appear white to human eyes but absorb ultraviolet strongly. Some appear dark but reflect infrared.

Emissivity matters alongside absorptivity. Hot objects emit infrared radiation—this is how heat radiates. Materials with high emissivity (like dull black surfaces) both absorb incoming radiation efficiently and emit their own heat radiation efficiently. This is why the most effective solar collectors use selective coatings that absorb visible and near-infrared light efficiently (like black) but emit infrared poorly (limiting heat loss)—a combination not achievable with simple paint.

Aluminium foil shows both low absorptivity and low emissivity—it reflects incoming light and emits very little radiation—which is why it's used in insulation (space blankets, attic insulation, thermos flask linings). It keeps things cool by not absorbing heat AND keeps things warm by not radiating it away.

Real-World Applications

Urban planners are increasingly using these principles to combat urban heat islands—cities that are several degrees warmer than surrounding countryside because dark roads, pavements, and rooftops absorb heat. "Cool roof" initiatives mandate light-coloured or reflective roofing materials in hot-climate cities, measurably reducing urban temperatures and air conditioning demand.

Space agencies use colour-based thermal management extensively. Spacecraft are covered in specific combinations of dark and reflective surfaces to maintain operational temperature ranges in the extreme environment of space. Temperature control on satellites, rovers, and space stations depends on precisely engineered surface emissivity and absorptivity.

Clothing design uses these principles practically. Many sportswear manufacturers now use infrared-reflective dyes that appear colourful to human eyes but reflect the near-infrared wavelengths that carry much of the sun's heat. This keeps athletes cooler without requiring white-only sportswear.

Taking It Further

Variation 1: Test damp versus dry coloured fabric. Does moisture affect heat absorption? (It does—wet dark clothing heats up slightly differently from dry dark clothing, relevant for agricultural and outdoor equipment design.)

Variation 2: Compare indoor versus outdoor performance. On a sunny day, measure whether jars near a sunny window warm as fast as jars in direct outdoor sunlight. Window glass blocks some wavelengths of solar radiation, affecting results.

Variation 3: Try the experiment at different times of day and measure how sun angle affects results. At 9am and at noon, the same black jar should warm at different rates because the sun's angle changes the intensity of light hitting the horizontal surface.

Related Questions to Explore:

Why do deserts stay cooler at night than forests, given they absorb so much heat during the day?
How do butterflies use the colour of their wings for thermoregulation?
Why does snow sometimes feel warm when you're lying in it?

Experiment 3: Make a Simple Solar Cell

What You'll Learn

Create a primitive photovoltaic cell using copper and table salt solution, demonstrating the photoelectric effect—the quantum mechanical phenomenon that makes solar panels work and earned Albert Einstein his Nobel Prize.

Equipment Needed

Two strips of copper sheeting, approximately 5cm × 10cm each (available from plumbing supplies or craft stores)
Sandpaper (fine or medium grade)
A gas hob, Bunsen burner, or camping stove
Tongs or pliers for holding metal
A large clear glass or plastic container (at least 1 litre)
Table salt (2-3 tablespoons)
Water (enough to fill the container)
Crocodile clip wires (2 pairs, available from any electronics shop)
A sensitive microammeter or galvanometer (measures very small currents; available from science supply shops or online for £5-15)
Optional: a multimeter (measures voltage and current; widely available)

Estimated cost: £10-20 (primarily for the copper strips and current meter)

Where to source: Copper strip from plumbing or craft suppliers. Crocodile clip wires and microammeter from electronics suppliers like RS Components, CPC, or Maplin. Some science toy shops stock complete solar cell kits.

Safety note: The heating step requires care with hot metal. Use tongs throughout—copper heats quickly and retains heat dangerously. Adult supervision essential. Do not touch heated copper. Ensure good ventilation.

Method

1. Prepare the copper strips (5 minutes) Sand both copper strips thoroughly with sandpaper to remove any oxidation or surface contamination. The copper should be bright and shiny. Clean surfaces are essential for the oxide layer to form properly.

2. Heat and oxidise one copper strip (15-20 minutes) Using tongs, hold one copper strip in a gas flame. Heat it to a high temperature—the copper will first turn dark (copper oxide forming) and then bright orange (the metal itself). Hold at high temperature for 2-3 minutes, ensuring the entire surface oxidises.

Remove from heat and allow to cool SLOWLY at room temperature (do not quench in water). As it cools, you'll see colours shifting—blue, purple, red—as different thickness oxide layers form. Finally, the copper should be covered in a thick, black layer of copper oxide (cupric oxide). This black oxide layer is crucial.

After cooling completely, gently brush the surface with your fingers or a soft cloth. Some of the black oxide will flake away, revealing reddish-copper oxide beneath. The outer black layer goes; the inner reddish layer remains. This inner layer (cuprous oxide) is your semiconductor.

3. Prepare your electrolyte solution (5 minutes) Fill your clear container with water and add 2-3 tablespoons of salt. Stir until dissolved. This saline solution will conduct electricity between your copper electrodes.

4. Assemble the cell (5 minutes) Attach crocodile clip wires to both copper strips. Place both strips in the saline solution, facing each other but not touching—one untreated (clean copper), one with the oxide layer (your treated strip). Both should be partially submerged with the clips above water level.

Connect the wires to your microammeter or multimeter.

5. Illuminate and measure (15-30 minutes) In a dark room initially, record your baseline current reading (it should be very small or zero). Now expose the treated (oxide-coated) copper strip to direct sunlight or bright light. Shine sunlight or a bright torch directly onto the oxide surface.

You should see a small but measurable current! The reading will be in microamps (millionths of an amp)—small but real, generated directly by light falling on your copper oxide semiconductor.

Try covering and uncovering the oxide strip with your hand, watching the current respond. Try different light sources—sunlight, incandescent bulb, LED lamp. Compare intensities.

Expected Results

You should observe a small photocurrent—typically 10-50 microamps—when bright light falls on the cuprous oxide surface. This current should increase with light intensity and decrease when the light source is removed or covered.

The current will be tiny compared to commercial solar cells. Your cell is inefficient—cuprous oxide's efficiency as a photovoltaic material is less than 2%, compared to 20-25% for modern silicon cells. But the principle is identical. You're generating electricity from light through the same quantum mechanical process as every commercial solar panel.

Troubleshooting: If you see no current, ensure the oxide layer on your treated strip is the reddish cuprous oxide, not just black cupric oxide (which shouldn't remain after gentle flaking). Your meter may not be sensitive enough—try a microammeter rather than a standard multimeter. If current is present in the dark, you have a simple galvanic cell (like a battery) from dissimilar metals in electrolyte—this is normal background, and your photocurrent should be visible above it when light shines.

The Science Explained

Your copper oxide solar cell demonstrates the photoelectric effect—the phenomenon that earned Albert Einstein the Nobel Prize in Physics in 1921 (though relativity gets more popular attention, it was the photoelectric effect that won him the prize).

The photoelectric effect occurs when light strikes a semiconductor and ejects electrons from their normal positions. Light isn't just a wave; it travels in discrete packets called photons. When a photon with sufficient energy strikes an electron in the semiconductor, it can knock that electron free, allowing it to flow as electric current. This is the fundamental process behind all photovoltaic cells.

Cuprous oxide (Cu₂O) is a p-type semiconductor—its crystal structure creates "holes" (missing electrons) that effectively carry positive charge. When sunlight (photons) strikes the cuprous oxide, it knocks electrons free, which flow through the external circuit (your wire and meter) from the clean copper to the oxide copper, creating measurable current.

Commercial silicon solar cells use the same principle but exploit a p-n junction—layers of p-type and n-type silicon that create an internal electric field, dramatically improving efficiency. Modern multi-junction cells with specialised semiconductor layers can exceed 40% efficiency in laboratory conditions by capturing different portions of the solar spectrum with different materials.

The quantum nature of light—the fact that it travels in photons with specific energies rather than as continuous waves—is why there's a minimum photon energy required to eject electrons. Red light (low-energy photons) is less effective than blue light (higher-energy photons) at generating photocurrent in many materials. This is why solar cells are designed to capture the entire solar spectrum, not just visible light.

Real-World Applications

Photovoltaic solar panels now generate significant electricity worldwide. In 2023, solar power provided approximately 5% of global electricity—a figure growing by 25-30% annually. The UK has installed over 15 gigawatts of solar capacity, enough to power 4 million homes.

The physics limiting solar cell efficiency—theoretical maximum efficiency for single-junction silicon cells is about 33% (the Shockley-Queisser limit)—drives research into multi-junction cells, quantum dot cells, and perovskite cells. Each approach tries to capture more of the solar spectrum or exploit quantum effects to exceed standard limitations.

Beyond electricity generation, the photoelectric effect has applications throughout technology. Photodetectors in cameras, optical communication systems, and scientific instruments all rely on the same principle as your copper oxide cell. Image sensors in every digital camera and phone use arrays of microscopic photovoltaic elements to convert light to electrical signals.

Taking It Further

Variation 1: Test different light sources—sunlight, incandescent bulb, fluorescent light, LED lamp. Do they produce different currents? The differences reveal how photon energy (light colour) affects the photoelectric response.

Variation 2: Try varying the area of oxide copper exposed to light. Double the area and see if current roughly doubles—exploring how solar cell output relates to collection area.

Variation 3: Build two cells and connect them in series (positive terminal of one to negative of other). Does the voltage double? Connect them in parallel. Does the current double? This demonstrates fundamental principles of solar array design.

Kitchen Science #6: Solar Science, 3 Illuminating Experiments to Harness the Power of the Sun

Leave a comment