Physics P6 Waves Flashcards
reflection at a surface
i
is the incident angle: the angle between the normal and the direction of the incident wave
r
is the reflected angle: the angle between the normal and the direction of the reflected wave
normal is perpendicular to surface
The law of reflection
incident angle = reflected angle
(Both rays and the normal must also lie in the same plane).
Reflection of crests and troughs
reflection from smooth surface
all the normals are parallel to one another so all the waves are reflected in an orderly way (e.g. reflection from a mirror) and images can be formed.
reflection from rough surface
the normals at each point are in different directions so each ray is reflected in a random direction (e.g. reflection from a white sheet of paper).
why does refraction at a boundary occur
When waves cross a boundary between two different media in which the waves travel at different speeds they refract.
changes in direction refraction at a boundary
if they are not travelling parallel to the normal then the wave direction changes as they cross the boundary.
If a light ray slows down it refracts toward the normal (e.g. air to glass).
If the waves are travelling along the normal they continue in the same direction (but still change speed).
Partial reflection and partial refraction
It is unusual for 100% of the incident wave energy to be reflected,
= some is likely to be absorbed by the surface material
= if the material is transparent, some will refract into the material.
does reflection change frequency and wavelength
no
does refraction change frequency
yes
ripple tank
used to demonstrate how water waves reflect and refract
The ripple tank shows the behaviour of wave crests.
Dark lines, parallel to wave crests, are seen on the screen.
Water waves and light waves reflect and refract in a similar way so we can use the behaviour of water waves to see how light waves behave and vice versa.
ray box
how light waves reflect and refract.
Ray box experiments show the behaviour of rays.
Water waves and light waves reflect and refract in a similar way so we can use the behaviour of water waves to see how light waves behave and vice versa.
Differences between water waves and light waves
The analogy between water waves and light waves cannot be taken too far. There are important differences between the two types of wave:
Light waves are electromagnetic and water waves are mechanical.
Light can travel through a vacuum and water waves cannot.
Light waves consist of vibrations of electric and magnetic fields and water waves consist of vibrations of particles.
Doppler effect
When there is relative motion between a source of waves and an observer, the wavelength and frequency of the waves detected by the observer is different from the wavelength and frequency of the waves received when there is no relative motion.
This change in wavelength and frequency is called the Doppler effect.
Source and observer approaching one another
Effect on wavelength = Shorter
Effect on frequency = Higher
