Reflection, refraction and lenses
What you'll learn
- How to draw and use ray diagrams for reflection and refraction.
- Why light changes direction at boundaries, including total internal reflection.
- How coloured surfaces and filters affect the light you see.
- How converging and diverging lenses form real and virtual images.
Most of the detailed ray-diagram and lens work in this part of Edexcel is marked with P, meaning it is Separate Physics content. Since you are studying 1PH0 Physics, it is on your course.
Starting point: light is an electromagnetic wave
Visible light is part of the electromagnetic spectrum. All electromagnetic waves are transverse waves and travel at the same speed in a vacuum: about 3.0×108 m/s3.0 \times 10^8\,\text{m/s}3.0×108m/s.
Transverse wave
A transverse wave has vibrations at right angles to the direction the wave transfers energy.
Electromagnetic waves transfer energy from a source to an observer. The observer might be your eye, a camera sensor, your skin absorbing infrared radiation, or an aerial receiving radio waves.
Rays and normals
A ray is a straight line showing the direction light travels. Ray diagrams are simplified drawings, but they are very useful because light often travels in straight lines through one transparent material.
Normal
The normal is an imaginary line drawn at 90° to a surface at the point where a ray hits it. Angles in reflection and refraction are measured from the normal, not from the surface.
Reflection
Reflection happens when light bounces off a surface. For a flat mirror, the reflected ray follows a simple rule called the law of reflection:
i=ri = ri=rwhere iii is the angle of incidence and rrr is the angle of reflection.
Law of reflection
The angle of incidence equals the angle of reflection, and both angles are measured from the normal.
Specular and diffuse reflection
Specular reflection happens from a smooth surface, such as a mirror. The reflected rays stay neatly organised, so you can see a clear image.
Diffuse reflection happens from a rough surface, such as paper or a wall. Each tiny part of the surface reflects light in a different direction, so no clear image forms.
Using the law of reflection
A ray hits a mirror with an angle of incidence of 35°. Find the angle of reflection and the angle between the incident and reflected rays.
- Apply the law of reflection: i=ri = ri=r, so r=35∘r = 35^\circr=35∘.
- The incident and reflected rays are on opposite sides of the normal, so the angle between them is 35∘+35∘=70∘35^\circ + 35^\circ = 70^\circ35∘+35∘=70∘.
- If you needed the angle between the reflected ray and the mirror surface, use 90∘−35∘=55∘90^\circ - 35^\circ = 55^\circ90∘−35∘=55∘.
Measuring from the surface
Do not measure the angle from the mirror surface. In ray diagrams, angles of incidence, reflection and refraction are measured from the normal.
Refraction
Refraction is the change in direction of light when it passes from one material into another because its speed changes.
A medium is a material that a wave travels through, such as air, water or glass. Glass is more optically dense than air, meaning light travels more slowly in glass than in air.
When light enters a more optically dense material, it bends towards the normal. When light enters a less optically dense material, it bends away from the normal.
Predicting the direction of refraction
A ray of light travels from air into glass at an angle to the normal. Predict how the ray bends.
- Compare the materials: light travels more slowly in glass than in air, so glass is more optically dense.
- Moving into a more optically dense material means the ray bends towards the normal.
- On the diagram, draw the refracted ray inside the glass closer to the normal than the incident ray.
Core practical: refraction in a rectangular glass block
In the core practical, you investigate how light refracts when it enters and leaves a rectangular glass block.
A typical method is:
- Place the glass block on paper and draw around it.
- Use a ray box to shine a thin ray into the block at a chosen angle.
- Mark the incident ray and the emergent ray.
- Remove the block, join the points where the ray entered and left, and draw normals at both surfaces.
- Measure the angle of incidence and angle of refraction with a protractor.
- Repeat for different angles of incidence.
For a rectangular block, the ray bends towards the normal as it enters, then away from the normal as it leaves. Because the opposite faces are parallel, the emergent ray is usually parallel to the incident ray, but shifted sideways.
Practical accuracy
Use a sharp pencil, keep the block still while tracing, draw the normal carefully at 90°, and never shine a ray box or laser into anyone’s eyes.
Total internal reflection
Total internal reflection, or TIR, happens when light reflects completely back inside a material instead of refracting out.
This can only happen when:
- light is travelling from a more optically dense material to a less optically dense material, such as glass to air
- the angle of incidence is greater than the critical angle
Critical angle
The critical angle is the angle of incidence where the refracted ray travels exactly along the boundary between the two materials.
Comparing incidence angle with critical angle
For a glass-air boundary, the critical angle is 42°. Predict what happens for incidence angles of 30°, 42° and 55°.
- Check the direction: the light is going from glass to air, so total internal reflection is possible.
- Compare each angle with the critical angle: 30∘<42∘30^\circ < 42^\circ30∘<42∘, so the ray refracts out into the air.
- At 42∘42^\circ42∘, the refracted ray travels along the boundary; at 55∘>42∘55^\circ > 42^\circ55∘>42∘, total internal reflection happens.
Forgetting the direction
Total internal reflection cannot happen when light travels from air into glass. It must be travelling from a more optically dense material into a less optically dense material.
Colour, absorption and filters
White light contains a mixture of colours. A surface appears coloured because it reflects some colours more than others and absorbs the rest. This is called differential absorption.
A red object in white light looks red because it reflects red light and absorbs many other colours. A black object absorbs most visible light, while a white object reflects most visible light.
A filter works by transmitting some colours and absorbing others. For example, a red filter transmits red light and absorbs much of the other visible light.
Combining a coloured filter and a surface
White light passes through a red filter and then shines on a blue surface. Predict how the surface appears.
- The red filter transmits mainly red light, so mostly red light reaches the surface.
- A blue surface reflects mainly blue light and absorbs many other colours.
- Since there is little blue light available to reflect, the surface appears very dark or black.
Lenses
A lens is a transparent object shaped so that it refracts light in a useful way.
A converging lens is thicker in the middle and brings parallel rays together. A diverging lens is thinner in the middle and spreads parallel rays out.
Focal point and focal length
The focal point is where parallel rays meet, or appear to come from, after passing through a lens. The focal length is the distance from the centre of the lens to the focal point.
Lens power
The power of a lens tells you how strongly it refracts light. It is linked to focal length by:
P=1fP = \frac{1}{f}P=f1where PPP is lens power in dioptres, D, and fff is focal length in metres.
A shorter focal length means a greater lens power. A more strongly curved lens usually has a shorter focal length and therefore a greater power.
Calculating lens power
A converging lens has a focal length of 0.20 metres. Calculate its power.
- Use the relationship P=1fP = \frac{1}{f}P=f1, with focal length in metres.
- Substitute the value: P=10.20 mP = \frac{1}{0.20\,\text{m}}P=0.20m1.
- Calculate the result: P=5.0 DP = 5.0\,\text{D}P=5.0D, so this is a fairly strong lens.
Ray diagrams for lenses
For lens diagrams, draw the principal axis, which is the straight line through the centre of the lens. Mark the focal points on both sides.
For a converging lens, useful construction rays are:
- a ray parallel to the principal axis refracts through the far focal point
- a ray through the centre of the lens continues straight
- a ray through the near focal point emerges parallel to the principal axis
For a diverging lens:
- a ray parallel to the principal axis leaves as if it came from the near focal point
- a ray through the centre of the lens continues straight
- dashed lines show where rays appear to have come from
A real image forms where light rays actually meet. It can be projected onto a screen.
A virtual image forms where light rays only appear to come from. It cannot be projected onto a screen.
Identifying a lens image
An object is placed between F and 2F in front of a converging lens. Describe the image formed.
- Draw a ray from the top of the object parallel to the axis; after the lens it passes through the far focal point.
- Draw a second ray through the centre of the lens; it continues straight and meets the first ray on the far side of the lens.
- Because the rays actually meet, the image is real. It is inverted and larger than the object.
Real versus virtual
If the real rays meet, the image is real. If you need dashed backward extensions to find where the rays appear to come from, the image is virtual.
In the exam
- Always draw the normal or principal axis first, then measure angles or place focal points from that reference line.
- For refraction and TIR questions, state both the direction of travel and whether the ray is moving into a more or less optically dense material.
- For lens diagrams, use two accurate construction rays and decide whether the rays really meet or only appear to meet.
Check yourself
- Why does a ray bend towards the normal when it enters glass from air?
- What two conditions are needed for total internal reflection?
- How can you tell from a lens ray diagram whether an image is real or virtual?
