Revision notes for AQA GCSE Physics Visible light (physics only). Open the guide for explanations and worked examples. Written against the AQA GCSE Physics (8463) specification, so the content matches what's examinable rather than general Physics background.
Visible light (physics only)
What you'll learn
- How each colour links to wavelength — the distance between matching points on a wave — and frequency — the number of waves passing per second.
- Why smooth surfaces give clear reflections, while rough surfaces scatter light.
- How colour filters work by transmitting some light and absorbing the rest.
- Why opaque, transparent and translucent objects appear the colours they do.
Visible light as part of the electromagnetic spectrum
The electromagnetic spectrum is the full range of electromagnetic waves, including radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays. Visible light is the small part of this spectrum that your eyes can detect.
A spectrum is a continuous range. The visible light spectrum contains colours from red through to violet. Each colour is not just “a name”: it has its own narrow band of wavelengths and frequencies.
Definition
Wavelength and frequency
- Wavelength, symbol λ\lambdaλ, is the distance from one wave crest to the next matching crest. It is measured in metres (m).
- Frequency, symbol fff, is the number of waves passing a point each second. It is measured in hertz (Hz).
For waves travelling at the same speed, frequency and wavelength are linked by:
v=fλv = f\lambdav=fλwhere vvv is wave speed in metres per second (m/s). In air, visible light travels at about 3.0×108 m/s3.0 \times 10^8\ \text{m/s}3.0×108 m/s, very close to its speed in a vacuum.
Red light has a longer wavelength and lower frequency. Violet light has a shorter wavelength and higher frequency.
Key Idea
Colour and wave properties
Each colour in visible light is a narrow band of wavelength and frequency. In the same medium, higher frequency means shorter wavelength.
Example
Calculating frequency from wavelength
A red light wave has wavelength 6.5×10−7 m6.5 \times 10^{-7}\ \text{m}6.5×10−7 m. Calculate its frequency in air using v=3.0×108 m/sv = 3.0 \times 10^8\ \text{m/s}v=3.0×108 m/s.
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Rearrange the wave equation to make frequency the subject:
f=vλf = \frac{v}{\lambda}f=λv -
Substitute the speed and wavelength, keeping the units with the numbers:
f=3.0×108 m/s6.5×10−7 m=4.6×1014 Hz\begin{aligned} f &= \frac{3.0 \times 10^8\ \text{m/s}}{6.5 \times 10^{-7}\ \text{m}} \\ &= 4.6 \times 10^{14}\ \text{Hz} \end{aligned}f=6.5×10−7 m3.0×108 m/s=4.6×1014 Hz -
This is a very high frequency, which is normal for visible light. A violet light wave would have an even higher frequency because it has a shorter wavelength.
Reflection: smooth surfaces and rough surfaces
A ray is a straight line with an arrow used to show the direction light travels. A normal is an imaginary line drawn at right angles to a surface where a ray hits.
When light hits a surface, it may be reflected, meaning it bounces off the surface.
Definition
Specular and diffuse reflection
Specular reflection is reflection from a smooth surface in one clear direction. Diffuse reflection, also called scattering, is reflection from a rough surface in many different directions.
A mirror gives specular reflection because its surface is smooth. Parallel incident rays reflect as parallel rays, so your eye can form a clear image.
Paper gives diffuse reflection because, close up, its surface is rough. Each tiny part of the surface has a different angle, so the reflected rays spread out in many directions. That is why you can see paper from many positions, but you cannot see a clear mirror-like image in it.
Common Mistake
Diffuse reflection is still reflection
A rough surface has not “stopped reflecting”. It still reflects light, but the reflected rays are scattered in many directions rather than staying neatly ordered.
Example
Predicting image formation from a surface
A lamp shines on a mirror and on a matte white wall. Explain which surface can form a clear image.
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A clear image needs rays from each point on the object to stay in a regular pattern after reflection.
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The mirror is smooth, so it produces specular reflection: the reflected rays remain ordered and can be traced back by your eye to form an image.
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The matte wall is rough, so it produces diffuse reflection: the reflected rays scatter in many directions, so no clear image forms, even though the wall is still visible.
Absorption, transmission and types of material
Light can interact with a material in three main ways.
Definition
Absorbing, transmitting and reflecting
- Absorbing light means taking in its energy rather than letting it pass through or bounce off.
- Transmitting light means letting it pass through.
- Reflecting light means sending it back from a surface.
An opaque object does not transmit visible light. You cannot see through it.
A transparent object transmits light with little scattering, so you can see a clear image through it. Clear glass is transparent.
A translucent object transmits light but scatters it, so light gets through but you cannot see a clear image. Frosted glass is translucent.
Key Idea
What decides the colour you see
The colour you see depends on which wavelengths of visible light finally reach your eye after reflection, transmission and absorption.
Colour filters
A colour filter is a material that transmits some wavelengths of visible light and absorbs others. For example, an ideal red filter transmits red wavelengths and absorbs most other visible wavelengths.
White light contains a mixture of visible wavelengths. Sunlight and many lamps produce light that your eyes interpret as white because many colours are present together.
The diagram shows the key difference: a filter selects what passes through, while an opaque object selects what is reflected.
Common Mistake
Filters do not add colour
A filter does not create new wavelengths. It can only transmit wavelengths that are already present and absorb the others.
Example
Light through two filters
White light passes through a green filter and then a red filter. Predict what light comes out.
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White light contains many visible wavelengths, including red and green.
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The green filter transmits mainly green light and absorbs much of the other visible light, including most red light.
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The red filter is now hit mainly by green light. Since an ideal red filter transmits red but absorbs green, very little light comes out, so it appears dark or black.
Why opaque objects have particular colours
For an opaque object, colour is mainly about reflection and absorption, not transmission.
A red opaque object appears red in white light because it reflects red wavelengths more strongly and absorbs many other wavelengths. A blue object reflects blue wavelengths more strongly. A green object reflects green wavelengths more strongly.
This is called differential absorption and reflection: different wavelengths are absorbed and reflected by different amounts.
Key Idea
Opaque object colour
An opaque object appears the colour of the wavelengths it reflects to your eye. Wavelengths that are not reflected are absorbed.
There are two important extremes:
- A white object reflects all visible wavelengths roughly equally.
- A black object absorbs all, or nearly all, visible wavelengths.
So a black object is not “reflecting black light”. It appears black because very little visible light reaches your eye from it.
Example
Viewing a green object through a red filter
A green object is viewed through a red filter in white light. Predict how it appears.
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The green object reflects mainly green wavelengths and absorbs many other wavelengths, including most red wavelengths.
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The light travelling from the object towards the filter is therefore mainly green.
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The red filter transmits red wavelengths but absorbs green wavelengths, so very little light reaches the eye. The object appears dark or black.
Tip
Trace the light path
For colour questions, follow the journey: light source → filter or material → object → filter or material → eye. At each stage, decide which wavelengths are transmitted, reflected or absorbed.
Transparent and translucent coloured objects
Objects that transmit light can also have colours.
A transparent red sheet looks red because it transmits red wavelengths more strongly than other wavelengths. You can still see a clear image through it if the transmitted light is not scattered much.
A translucent coloured plastic sheet may also transmit mainly one colour, but it scatters the transmitted light, so objects behind it look blurred.
Exam technique
In the exam
- Start with the light source: decide whether white light or only certain colours are present.
- For each filter or transmitting object, state which wavelengths are transmitted and which are absorbed.
- For an opaque object, state which wavelengths are reflected and which are absorbed, then conclude what reaches the eye.
Self review
Check yourself
- Why does a mirror form a clear image, but white paper does not?
- What would a blue object look like when viewed through a red filter in white light?
- Why does a black object appear black under white light?
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