This grade 7 science lesson on reflection of light notes gives the key ideas first, then worked examples and diagrams. After that, practise with the worksheet on reflection of light and check yourself with the quiz.
Quick answer: Light is reflected, refracted or absorbed when it meets matter. The law of reflection says the angle of incidence equals the angle of reflection, both measured from the normal. Refraction bends light towards the normal when it slows down in glass or water. We see objects because they reflect light into our eyes.
Level: Grade 7 science (Year 8 · Class 7) · Curriculum: MS-PS4-2
Reflection of light notes: the key ideas
How do we see objects?
A lamp, a candle or the Sun is a luminous source: it makes its own light. Most things around you, such as a book, a tree or a friend, are not luminous. You can see them only because light from a source bounces off them and enters your eye.
Reflection is the bouncing of light off a surface. Refraction is the bending of light as it passes from one transparent material into another, for example from air into water.
What is the law of reflection?
To describe a reflection we draw a dashed line at right angles to the surface at the point where the ray lands. This line is called the normal. The ray coming in is the incident ray and the ray leaving is the reflected ray. Angles are always measured from the normal, not from the surface.
Law of reflection: the angle of incidence i equals the angle of reflection r, i = r. The incident ray, the reflected ray and the normal all lie in the same flat plane.
A ray hits a plane mirror and makes an angle of 35° with the mirror surface. Find the angle of incidence and the angle between the incident and reflected rays.
The normal is 90° from the surface, so i = 90° – 35° = 55°. Then r = 55°, and the angle between the rays is 55° + 55° = 110°. i = 55°, angle between rays = 110°
In a plane (flat) mirror the image appears as far behind the mirror as the object is in front of it. It is the same size, upright and reversed left to right, and it cannot be caught on a screen because the light only seems to come from behind the mirror.
Emma in Leeds stands 1.5 m in front of a plane mirror. How far is she from her image?
The image is 1.5 m behind the mirror, so the distance is 1.5 m + 1.5 m. 3.0 m
What is the difference between regular and diffuse reflection?
A very smooth surface reflects parallel rays in one direction, which gives a clear image. A rough surface has many tiny faces tilted differently, so each ray still obeys i = r, but the reflected rays scatter everywhere. That scattering lets us see paper from any side.
| Surface | Type of reflection | What you see |
|---|---|---|
| Mirror, still water | Regular (specular) | A clear image |
| Paper, wall, cloth | Diffuse (scattered) | The object, from any angle, but no image |
What happens when light enters glass or water?
Light travels at 3.0 × 10⁸ m/s in air but more slowly in glass, about 2.0 × 10⁸ m/s. When a ray enters a slower material at an angle, one side of the beam slows first and the ray bends. Entering glass or water, it bends towards the normal; leaving, it bends away. A ray along the normal is not bent. This is why a straw looks broken at the water surface.
Light crosses a glass block 0.30 m thick at 2.0 × 10⁸ m/s. How long does it take?
t = d ÷ v = 0.30 ÷ (2.0 × 10⁸) = 1.5 × 10⁻⁹ s. t = 1.5 × 10⁻⁹ s
What happens to light that is not reflected?
When light hits an object it can be reflected, transmitted or absorbed. Clear glass transmits most light. A black T-shirt absorbs most visible light, which is why it feels hot in the Sun. A red book reflects mostly red light and absorbs the rest, so it looks red.
Do not measure the angle of incidence from the mirror surface. Always measure from the normal. If a ray makes 30° with the mirror, the angle of incidence is 60°.
Draw the normal first, in dashed pencil, every time. Then label i and r and check that both angles touch the normal. Most lost marks come from skipping this step.
- We see non-luminous objects because they reflect light into our eyes.
- Law of reflection: angle of incidence i = angle of reflection r, both measured from the normal.
- A plane mirror image is as far behind the mirror as the object is in front.
- Light entering glass or water slows down and bends towards the normal.
- Dark surfaces absorb more light energy; rough surfaces scatter light (diffuse reflection).
How light bounces, bends and is absorbed: frequently asked questions
What is the law of reflection?
The angle of incidence equals the angle of reflection, i = r. Both angles are measured from the normal, the line at right angles to the surface.
Why does light bend when it enters water?
Light travels more slowly in water than in air. When a ray enters at an angle, the change of speed makes the ray change direction, bending towards the normal.
What is the difference between reflection and refraction?
In reflection light bounces back from a surface into the same material. In refraction light passes into a second material and changes direction because its speed changes.
Why can we see a book that gives off no light?
A book reflects light from a lamp or the Sun in all directions, and some of that light enters our eyes. Without a light source we cannot see it.
Why does a black surface get hotter than a white one in sunlight?
A black surface absorbs most of the light energy that hits it, which warms it. A white surface reflects most of the light, so less energy is absorbed.
