Wave theory
Reflection
The law of reflection: the angle of incidence (the angle between the normal and the incident ray) must be the same as the angle of reflection (the angle between the normal and the reflected ray) on the other side.
How to draw ray diagrams?
- Draw a normal (imaginary dotted line which is perpendicular to the surface of the mirror.
- The incident light ray (the one coming in) must reach the mirror at the same point that the normal does.
- The light ray will be the one reflected off obeying the law of reflection.
Refraction
Refraction: when light travels from air into glass, it slows down. It speeds up again as it leaves glass and re-enters air. This change in speed causes the light ray to bend.
Angle of incidence > angle of refraction
Light travelling from air into a denser material will cause the light ray to bend towards the mirror.
Angle of incidence < angle of refraction
As light leaves glass and re-enters air, the light ray will bend away from the normal.
HOWEVER: If the light ray is travelling at 90 degrees to the boundary, it will not refract and just goes straight through.
What happens when a wave is refracted by entering a more dense medium?
Speed reduces.
Wavelength increases.
Frequency stays the same.
Refraction through a prism
The amount light bends when passing through the boundary between two materials depends on the wavelength of the light -> longer wavelength light bends through a smaller angle.
White light is a mixture of all the colours of the rainbow, so when it refracts, the different colours all bend a different amount and therefore spread out. This is called dispersion.
Red light refracted least.
Blue light refracted most.
Because of the shape of the prism, the incident white light refracts the same way twice and the dispersion can be seen clearly.
Refractive Index
Definition: a ratio of the sin(incidence angle) to the sin(refracted angle) when light moves across a boundary.
It tells us how much a ray of light will bend or the speed of light in the material.
Critical angle and Total Internal Reflection (TIR)
A ray of light that is being refracted away from the normal will eventually be bent so much that it travels along the boundary.
If any incident angle is bigger than the critical angle, this will cause the light to stay in the block. This is called total internal reflection.
Lenses
Converging (convex)
Diverging (concave)
Parallel rays are focused onto one point (focal point F). The distance to this point is called the focal length (f).
During refraction, the rays in a convex lens converge.
In refraction, when parallel light rays bend towards the normal when they enter a denser medium and away from the normal when they enter a less dense medium.
How does the refractive index of a lens affect its focal length?
Focal length: the distance in metres from the centre of the lens to the point where the image is formed.
The higher the refractive index -> the more refraction takes place -> focal point will be closer to the lens -> reduce the focal length
The lens with the higher refractive index will have shorter focal length.
Parallel rays are spread out; they appear to have come from one point. Since they haven't the rays on the left of the lens are virtual (dotted line).
During refraction, they rays in a concave lens diverge.
How does the shape of a lens affect its focal length?
A fat lens (more powerful) have a shorter focal length than a thin lens.
Fatter lenses have a lower curvature which makes them refract the light more, giving them a shorter focal length and more power.
Thin lenses have a greater curvature that makes them refract the light less, giving a longer focal length and less power.
How to describe the nature of images formed by lenses?
- Size: diminished and magnified.
- Orientation: upright and inverted.
- Type of image: real and virtual.
Waves
Longitudinal
Transverse
Longitudinal waves make particles oscillate in a direction parallel to the direction of energy transfer.
Examples: sound waves.
Transverse waves make particles oscillate in a direction perpendicular (at 90 degrees) to the direction of energy transfer.
Example: water waves, waves on a string and electromagnetic waves.
How to describe waves?
Wavelength: the distance between any two identical points on a wave (the length of one full cycle). Normally measured in metres. (long wavelength = loud; short wavelength = low)
Amplitude: the distance from the middle of the wave to the top or the bottom of the wave.
Frequency: this is the number of complete waves which pass a point in 1 second. Measured in Hertz (Hz) which means 'per second'. (high frequency = high pitch; low frequency = low pitch)
Sound waves
Caused by vibrations. These vibrations squash and stretch the air immediately next to it, and then they bump into their neighbours and so on.
Loudness: amplitude represents the loudness of the wave (larger amplitude = louder)
Pitch: frequency represents the pitch of the wave (higher frequency = higher pitch)
Electromagnetic spectrum
Humans range: 20Hz - 20,000Hz
Speed: they all travel at the speed of light (300,000,000 m/s).