Visible Light and Colour

(Separate Physics only) This topic is assessed only in the separate GCSE Physics course, not in Combined Science.

Visible light is the narrow band of the electromagnetic spectrum that the human eye can detect. Within it, each colour has its own narrow band of wavelength and frequency.

Red light has the longest wavelength and the lowest frequency of the visible colours. Violet light has the shortest wavelength and the highest frequency. The other colours lie in between, in the familiar order.

Colour	Wavelength	Frequency
Red	Longest	Lowest
Orange, yellow, green, blue	In between (decreasing wavelength)	In between (increasing frequency)
Violet	Shortest	Highest

White light is a mixture of all these colours together. Splitting it, for example with a prism, separates it into the spectrum from red to violet.

Because each colour is a different wavelength, materials can treat different colours differently, and that is what produces the effects covered in the rest of this lesson.

(Separate Physics only)

When light reflects off a surface, the type of reflection depends on how smooth the surface is.

Specular reflection happens at a smooth surface, such as a mirror or still water. Parallel rays of light are all reflected in the same single direction, so the surface produces a clear image. The angle of incidence equals the angle of reflection at every point, and because the surface is flat the reflected rays stay parallel.

Diffuse reflection happens at a rough surface, such as paper or a painted wall. The surface is made of tiny facets pointing in different directions, so parallel rays are reflected (scattered) in many different directions. No clear image forms, but the object can be seen from many angles.

Feature	Specular reflection	Diffuse reflection
Surface	Smooth (mirror, still water)	Rough (paper, wall)
Reflected rays	All in one direction	Scattered in many directions
Result	Clear image	No clear image; visible from all angles

In both cases the law of reflection still holds at every point (angle of incidence = angle of reflection). Diffuse reflection is not the law breaking down; it is many tiny smooth facets facing different ways.

(Separate Physics only)

A colour filter is a transparent material that changes the colour of the light passing through it. A filter works by absorbing some wavelengths and transmitting others.

A filter transmits its own colour and absorbs the other colours. So:

• A red filter transmits red light and absorbs all other colours. White light passing through it comes out red.
• A green filter transmits green light and absorbs the rest, so white light comes out green.

Worked reasoning. Shine white light (all colours) through a blue filter. The filter transmits blue and absorbs red, green and the rest. Only blue light emerges. Now shine that blue light through a red filter. The red filter absorbs blue, so almost no light gets through and the result looks black (dark). Two filters of different colours together can block nearly all the light.

A filter never adds colour. It can only remove (absorb) some of the colours already present in the light.

(Separate Physics only)

An opaque object is one that does not let light pass through it. Its colour is decided by which wavelengths of light it reflects and which it absorbs.

The colour of an opaque object is the colour of light it reflects most strongly; the other colours are absorbed.

• A red book reflects red light and absorbs the other colours, so it looks red.
• A white object reflects all colours of light equally, so it looks white.
• A black object absorbs all colours and reflects almost none, so it looks black.

The colour you see also depends on the light shining on the object.

Worked example. A red jumper is lit only by green light. The jumper reflects red but absorbs green; since there is no red light present to reflect, almost no light comes back from it, so the jumper looks black (dark). Shine white light on it again and it reflects the red part, so it looks red once more.

To answer these questions, compare the colours present in the light with the colours the object reflects. Only colours that are both present and reflected reach your eye.

(Separate Physics only)

Not all objects are opaque. Some let light pass through, and this changes how their colour is produced.

• A transparent object, such as clear glass or clean water, transmits light through it, and you can see clearly through it.
• A translucent object, such as frosted glass or tracing paper, transmits some light but scatters it, so light passes through but objects behind cannot be seen clearly.

For a transparent or translucent coloured object, the colour depends on which wavelengths it transmits. A piece of red transparent plastic transmits red light and absorbs the others, just as a red filter does, so it looks red and things seen through it look red.

Material	Behaviour with light	Example
Opaque	Reflects and absorbs; no light passes through	Wood, metal
Transparent	Transmits light clearly	Clear glass, water
Translucent	Transmits but scatters light	Frosted glass, tracing paper

The key distinction: an opaque object's colour comes from reflection, while a transparent or translucent object's colour comes from transmission.

1. Saying diffuse reflection breaks the law of reflection

The law of reflection holds at every point on a rough surface. The rays scatter because the tiny facets face different directions, not because the angles are unequal. State that the surface is rough, so parallel rays are reflected in many directions.

2. Thinking a filter adds colour to light

A colour filter can only absorb some colours and transmit others. It never creates a colour that was not in the original light. Two different filters together transmit little or no light.

3. Saying an object "is" a colour regardless of the light

An object's apparent colour depends on the light shining on it. A red object in green light looks black, because there is no red light for it to reflect. Check which colours are present in the light before deciding the apparent colour.

4. Confusing absorb and reflect for opaque colour

An opaque object shows the colour it reflects most and absorbs the rest. A black object absorbs all colours; a white object reflects all colours. Getting absorb and reflect the wrong way round reverses the answer.

5. Mixing up transparent and translucent

Transparent lets you see clearly through it; translucent lets light through but scatters it, so you cannot see clearly through it. Opaque lets no light through at all. Keep the three terms distinct.