Experiment 1: light travels in a straight line
You need: three cardboard cards, a hole punched in the same place on each, a torch, clay to stand the cards up.
Do: Stand the cards in a row with the holes in a line. Shine the torch through the first hole. The light comes out of the last hole. Now move the middle card a little. The light stops.
It shows: light moves in a straight line. It cannot go round a corner. That is why we get a shadow behind an object: the object blocks the straight rays.
Experiment 2: reflection in a mirror
Reflection means light bounces back from a surface. Draw a line at 90° to the mirror. This line is the normal. The ray that hits the mirror is the incident ray; the ray that bounces off is the reflected ray.
Measure the angles from the normal. You always find: angle of incidence = angle of reflection. The incident ray, the normal and the reflected ray lie in one flat plane.
Image in a plane mirror: the image looks as far behind the mirror as you stand in front. It is the same size, upright, and left and right are swapped. It is a virtual image: you cannot catch it on a screen, because light only seems to come from it.
Experiment 3: refraction in glass
Refraction means light bends when it goes from one clear material to another, for example from air into glass or water. Light is slower in glass than in air. That is why it turns.
Going from air into glass, the ray bends toward the normal. Going from glass into air it bends away from the normal. After a glass block with flat parallel sides, the ray comes out parallel to the first ray, only shifted sideways.
A ray that hits the surface straight on (along the normal) does not bend at all.
Experiment 4: convex lens and the focus
A convex lens is thicker in the middle. It bends parallel rays inward so they meet at one point, the focus (F). The distance from the lens to the focus is the focal length. A fatter lens bends more, so its focal length is shorter.
You can find the focal length at home: hold a magnifying glass so that sunlight makes the smallest, brightest dot on paper. The distance from lens to paper is about the focal length. (Do not leave the dot on paper; it can burn.)
A convex lens can make a real image that lands on a screen, like the picture inside a camera. A concave lens (thin in the middle) spreads rays out instead.
Experiment 5: white light in a prism
A prism is a glass block with triangle ends. White light entering one side bends, and bends again leaving the other side. Each colour bends by a different amount, so they spread out into a band: red, orange, yellow, green, blue, violet. This is dispersion. Red bends least; violet bends most.
So white light is a mix of colours. A rainbow is dispersion by tiny raindrops.
Try it: kitchen optics
1) Put a coin in an empty bowl and step back until the rim hides it. Ask a friend to pour water slowly. The coin appears: refraction lifts it into view. 2) Stand a pencil in a glass of water and look from the side. It looks bent. 3) In the 3D, guess where the red and violet rays will end before moving the slider. Check your guess.
Key formulas and definitions
- Straight line: light cannot bend round a corner; objects make shadows
- Reflection: angle of incidence = angle of reflection (measured from the normal)
- Plane mirror image: virtual, upright, same size, as far behind as the object is in front
- Refraction: air to glass bends toward the normal; glass to air bends away
- Convex lens: parallel rays meet at the focus; fatter lens = shorter focal length
- Prism: white light splits into 7 colours; red bends least, violet most
Worked examples
1. A ray hits a plane mirror at 35° to the normal. Find the angle of reflection and the angle between the incident and reflected rays.
Angle of reflection = angle of incidence = 35°. The angle between the two rays = 35° + 35° = 70°.
2. You stand 1.5 m in front of a plane mirror. How far are you from your image?
The image is 1.5 m behind the mirror. Distance from you to the image = 1.5 + 1.5 = 3 m.
3. A ray goes from air into glass at 30° from the normal. The ray inside the glass makes about 19° with the normal. Does it bend toward or away from the normal? Why?
19° is smaller than 30°, so it bends toward the normal. Light is slower in glass, and a ray entering a slower material bends toward the normal.
4. A magnifying glass makes the smallest bright dot of sunlight on paper when held 10 cm above it. What is its focal length? What would a fatter lens give?
The sunlight rays are almost parallel, so the dot is at the focus. Focal length is about 10 cm. A fatter lens bends rays more, so its focal length would be shorter than 10 cm.
Common mistakes
- Measuring angles from the mirror surface. Always measure from the normal (the line at 90° to the surface).
- Thinking the image in a plane mirror is on the mirror. It looks as far behind the mirror as the object is in front.
- Believing a prism adds colour to light. It only separates the colours that white light already has.
- Thinking light bends the same way going in and out of glass. Going in it bends toward the normal; coming out it bends away.