Section 1 of 9Refraction of light
Refraction
Refraction the change in direction of light when it passes a boundary between two transparent materials, caused by a change in speed.
Into a denser medium
- Into a denser medium light slows down and bends towards the normal.
- (remember: enters towards)
Into a less dense medium
- Into a less dense medium light speeds up and bends away from the normal.
- (remember: leaves away)
Along the normal
- Along the normal light does not bend.
Unchanged property
- Unchanged property the frequency, and so the colour, does not change, but the speed and wavelength do.
Section 2 of 9Rectangular glass block
Two boundaries
- Two boundaries the refracted ray at the first boundary is the incident ray at the second.
Result
- Result the emerging ray is parallel to the original ray but shifted sideways.
1
Draw around a rectangular block on paper and aim a ray from a ray box at one side face.
2
Mark a point on the incident ray, the entry point, the exit point and a point on the emergent ray.
3
Remove the block and join the points with straight lines.
4
Draw the normal at each boundary and measure the angles of incidence and refraction.
5
Repeat for several angles and for semicircular blocks and prisms.
Section 3 of 9Refractive index
Refractive index (n)
Refractive index (n) a number greater than 1 with no unit that shows how much a material slows and bends light.
From speeds
- From speeds n = speed of light in a vacuum ÷ speed of light in the material, so n = c ÷ v, with c = 3.0 × 108 m/s.
From angles
- From angles n = sin i ÷ sin r for light entering the material from air.
Typical values
- Typical values water 1.33, glass about 1.5 and diamond about 2.4.
Optical density
- Optical density a material with a higher refractive index is more optically dense.
Section 4 of 9Total internal reflection
1Total internal reflection (TIR)
Total internal reflection (TIR) all of the light in a dense medium is reflected back at the boundary with a less dense medium.
2Conditions
- Conditions the light must travel from the denser to the less dense medium, and the angle of incidence must be greater than the critical angle.
3Small angles of incidence
- Small angles of incidence most light is refracted out and a weak ray is reflected, which is only partial internal reflection.
4Brightness
- Brightness the totally reflected ray is brighter than an ordinary reflected ray.
Section 5 of 9Critical angle
Critical angle (c)
Critical angle (c) the angle of incidence in the denser medium for which the angle of refraction is exactly 90°.
Below c
- Below c the light refracts out of the denser medium.
At c
- At c the refracted ray travels along the boundary.
Above c
- Above c total internal reflection occurs.
Equation
- Equation sin c = 1 ÷ n, so n = 1 ÷ sin c.What the symbols mean: c = critical angle · n = refractive index of the denser material (no unit)
Effect of n
- Effect of n the larger the refractive index, the smaller the critical angle.
Section 6 of 9Uses of total internal reflection
Optical fibres
- Optical fibres light is totally internally reflected each time it hits the edge of the fibre and so travels along it.
- They carry telephone, internet and cable television signals as light pulses.
Endoscope
- Endoscope a bundle of fibres carries light into the body and brings the image back to the eyepiece.
Prisms
- Prisms right-angled prisms reflect light in periscopes, binoculars, cameras and safety reflectors.
Periscope
- Periscope two right-angled prisms totally internally reflect the light so that an observer can see over an obstacle.
Section 7 of 9More from the specification
More from the specification
Reflection and refraction
- Reflection obeys angle of incidence equals angle of reflection;
- refraction is a change of direction as light changes speed between media.
Plane mirror images
- The image is virtual, upright, laterally inverted and as far behind the mirror as the object is in front.
Converging lens
- Parallel rays are brought to the principal focus; the focal length is the distance from lens centre to focus.
Diverging lens
- Rays spread apart as though from a virtual principal focus, always producing a virtual image.
Image types
- A real image can be projected on a screen, a virtual image cannot;
- a converging lens used as a magnifying glass gives a virtual image.
Virtual image
- An image formed where rays only appear to come from, which cannot be projected.
Principal focus
- The point where rays parallel to the principal axis meet or appear to come from.
Focal length
- The distance from the centre of the lens to the principal focus.
Plane mirror
- A flat mirror.
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Refraction | The change in speed, and usually direction, of a wave crossing a boundary between media. |
| Refractive index | The ratio of the speed of light in a vacuum to its speed in a material, n = c ÷ v. |
| Critical angle | The angle of incidence in the denser medium at which the refracted ray travels along the boundary. |
| Total internal reflection | The complete reflection of light inside a denser medium when the angle of incidence exceeds the critical angle. |
| Optical fibre | A thin glass fibre that carries light by total internal reflection. |
| Optical density | A measure of how much a medium slows light, shown by its refractive index. |
Section 9 of 9Exam technique
1
Saying light bends away from the normal on entering glass — it bends towards the normal.
2
Saying frequency changes in refraction — frequency is constant while speed and wavelength change.
3
Saying TIR can happen going from air into glass — the light must be in the denser medium.
4
Giving refractive index a unit — it is a ratio and has no unit.
5
Using the angle with the surface instead of the angle with the normal.