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Camera obscura and lenses

Public summary: 

The camera obscura uses a lens to project an image of the outside world onto a white screen inside the CHaOS event. Can you figure out why it's upside down?

Peering at the outside world through a lens.
Useful information
Kit List: 

- Camera obscura lens in cardboard sheath, should be packed carefully to avoid scratches
- Wooden board painted white
- Ray box and 2 colour filter light boxes
- Power supply for lights
- Lenses ((i) "eye" lens, (ii) corrective lens(es)), should be packed carefully to avoid scratches
- Sheets of A4 white paper to go under ray boxes
- Sheet of A4 paper to act as screen for ray boxes
- Poster showing cutaway diagram of eye

Packing Away: 

The Lens often live in the lens box, camera obscura occasionally get placed in misc when made out of pringles tubes.

Frequency of use: 


This is a large experiment with 3 parts.

1. A glass camera obscura lens in the side of the darkroom tent, and a wooden board. If you hold the board in the correct place inside the darkroom then an upside down image of the outside world will be projected onto it through the lens.

2. Ray boxes and lenses on a table. This is essentially a 2d representation of the camera obscura which you can use to explain it. You can show: why the image is upside down; why the lens is important; what focussing does; and how corrective lenses work in the eye.

3. Poster showing cutaway diagram of eye. Allows you to link in the bio factors, talk about the parts of the eye, nerves etc.

Set Up

- Position the darkroom, and the camera obscura lens, such that it is looking out on a brightly lit, active area with, if possible, plenty of depth for varying focus. Ideally it's a good idea not to let people (parents/ demonstrators) congregte just in front of the lens... out of focus legs blocking most of the picture aren't very exciting!

- Cut a hole for the lens at about child's eye level (or find a suitable hole), and tape the lens and sheath in place.

- Set up the ray box and light boxes on the table, and check they work. They should have switches on them.


- Get the 2 sheets of paper showing the eye with/without corrective lenses and tape the light boxes in the correct place.


- Tape up the poster of the eye somewhere


(This is one approach to demonstrating this experiment. It's quite long so it might be wise to split it between 2 people.)

1. The camera obscura

1a. Encourage children to look through camera obscura lens from various distances, observing that they can see what's outside, but blurry and possibly upside down.

1b. Either holding the white board yourself, or getting the children to hold it, position the board so that the image is in focus on it. Ask them what they can see and get them to realise it's an image of outside.

1c. Ask them what's wrong with the image. Eventually they'll get that it's upside down. Try turning the board over and show that it doesn't make a difference. If one is wearing a white T-shirt, try getting the image focused on them, if not, use your arm... make it clear there's nothing special about the paper- it's the light coming in.

1d. Move the board around and show that the image is blurry unless it's in the right position.

2. The eye

(This seems to be well received by everyone, and is fairly interactive but not hands-on. There's loads you can talk about: cameras, eyes, shutter speeds, focussing, digital cameras, nerves. This is one approach).

2a. Say that this is how your eye works. Shock horror. DON'T move over to the eye poster yet - it seems to distract them and it's pretty dull. Instead, talk about it while you're holding the camera obscura board and point with your arm to where the eyeball would be.

2b. Start by talking about the pupil - "what's in the middle of your eye?" - most people know what its called and what colour it is so this is a good starting point. Then say its a hole that lets light into the eye, just like the hole in the side of the tent. Again, point with your arm to where the eyeball and pupil would be in the tent, relative to the board that you're holding.

2c. A hole isn't much use by itself, its the glass thing that does the work. You've got one in your eye too. Yes really. What's it called? A lens. What's it made of? Glass. How do you make glass? Heat sand to high temperatures. So can you do this in your body? No. So your body uses jelly to make the lens instead.

2d. Ask them what the eye is filled with. The eye is filled with clear jelly liquid. does light go through water? Yes. So (pointing with your arm again) the light goes through the hole into your eye until it hits the other side of the eye.

2e. So the light goes through the hole into your eye until it hits the other side of the eye. What happens on the other side of the eye? You get an image upside down, just like on the wooden board for the camera obscura.

2f. That's great but you still can't see. The neat trick is to cover the board at the back of the eye with electrical sensors that sense what colour light is hitting each point on the board. So here they say "yellow", here they say "red", etc. All this gets sent down wires to the brain so you can see.

2g. Now move over to the poster of the eye diagram and show them that it's all true. The orientation of the cutaway diagram isn't obvious at first so you have to help them with this.

2h. So, on to these wires. What're they called? Nerves. Have you seen wires in the house? What're they made of? The middle bits made of metal. It conducts. Do you think wires in the body are made of metal? No. What else conducts? Salt water. So the body uses salt water instead. What about the plastic? It insulates. Do you have plastic in your body? No. So use fat instead.

3. Ray boxes

(Younger children tend not to receive this part very well - they just don't grasp the correspondence between the 3d camera obscura and the 2d ray boxes. However there's still some mileage in shadows and light going in straight lines. If you've been talking to them for ages already it's better to get someone else to demonstrate this bit otherswise they tend to get bored.)

3a. Move to ray-box table, tell them we're going to explain why the image is upside down. Ask if they have any thoughts about it.

3b. You could show light travelling in straight lines- talk about shadows- caused because light can't go round corners, etc.

3c. Now with the lens in place, compare the system you have to the camera obscura - eg "we have the light coming in on this side, and here's the lens, etc...". Hold a piece of white paper up as "the board".

3d. Point out that the image is upside down - on one side of the lens, the red light is at the top and the green light is at the bottom, but on the "board" the reverse is true. Get them to work out that the lights cross over inside the lens, so the red light starts out at the top and goes to the bottom, and vice versa for the green light.

3e. Compare this back to the camera obscura, where light from the ceiling ends up at the bottom of the board and light from the floor ends up at the top of the board. The light crosses over inside the lens.

3f. Talk about focussing the light to a point by moving the "board". Relate this back to moving the board for the camera obscura.

3g. If they're already familiar with how the eye works then you could take them through corrective lenses. Use a mishaped eye (squashed black circle on white paper) with one lens and show that the light doesn't focus properly on the edge of the circle. Now add a corrective lens in front of that lens and show that it now focuses properly. Get them to relate this to contact lenses and glasses.

Risk Assessment
Date risk assesment last checked: 
Mon, 03/02/2020
Risk assesment checked by: 
Date risk assesment double checked: 
Mon, 03/02/2020
Risk assesment double-checked by: 
Beatrix Huissoon
Risk Assessment: 

Using a lens in a dark room:

Hazard Risk Affected Person(s) Likelihood Severity Overall Mitigation Likelihood Severity Overall
Glass lens (breakages) Broken glass shards may cut people. All 4 4 16 Make sure the lens won't fall on floor, e.g. by using tape. Clear up broken glass immediately and keep people away while doing so. Call a first aider in the event of an injury. 2 3 6
Glass lens (positioning) If lens is positioned in direct sunlight, then the focussed sunlight can cause retinal damage and burns All 2 4 8 Demonstrator to make sure that the lens is not positioned in direct sunlight. Call a first aider in the event of an accident. 1 4 4

Use a ray box to observe light paths :

Hazard Risk Affected Person(s) Likelihood Severity Overall Mitigation Likelihood Severity Overall
Electrical parts Electrical hazard (see Electrical Parts RA). All 3 5 15 Use low voltage equipment (3V). Power supply should be PAT tested, as per the electrical parts RA. 1 3 3
Bulbs Bulbs become very hot when in use, box may become hot over time, possibly leading to burns. All 4 3 12 Some bulbs are enclosed in plastic case. Do not remove case. Do not touch bulbs. Bulbs not in plastic cases have been tested to ensure they do not get too hot. Turn box off when not in use. Call a first aider in the event of an accident. 2 2 4
Prisms, lenses and bulbs (glass) Prisms, lenses and bulbs may shatter if dropped on floor - possibility of cuts. All 4 4 16 Discourage rough handling of lenses and prisms. Clear up broken glass immediately and keep people away while doing so. Call a first aider in the event of an injury. 2 3 8
Electrical cables Trip hazard from cable All 5 2 10 Keep the cable to the ray box out of reach if possible. If cable is somewhere it might present a trip hazard, ensure it is taped down. Call a first aider in the event of an accident. 3 2 6
This experiment contains mains electrical parts, see separate risk assessment.
This experiment is sometimes run in a darkroom, see separate risk assessment.
Publicity photo: 
Experiment photos: