P4: Waves (Shi Yu) (Light (Refraction ( Refractive index of a medium:
P4: Waves (Shi Yu)
- Refractive index of a medium:
n = speed of light in a vacuum / speed of light in the medium
n = sin i / sin r
- As the ray of light enters the glass block it shows down and bents toward the normal so i > r.
- As the ray of light leaves the glass block it speed up and bents away the normal.
4.The frequency of light is unchanged as it travels from one medium to another. As the light speeds up the wavelength longer. As the light slows down the wavelength shorter.
- If i is 90度, the direction unchanged.
1.As the angle of incidence increase, the angle of refraction also increase, until r = 90 . When r = 90 , the angle of incidence = the critical angle.
- If the angle of incidence is increased beyond the critical angle, total reflection occurs.
3.Total internal reflection can only occur when the incident angle is larger than the critical angle.
Thin converging lens
- Converging lens:
1) When parallel ray pass through a convex converging lens, the are focused to principal focus.
2) The principal focus lies on the principal axis.
3) The distance between the principal focus and the centre of the lens is called the focal length.
- Ray diagram:
1) Object is located beyond 2F:
Between F and 2F, its inverted, reduced, real.
2) Object is located at 2F:
At the 2F, its inverted, same size, real.
3) Object is located between F and 2F:
Beyond 2F, its inverted, magnified, real.
4) Object is located F:
No Description Required
5) Object is located between F and the lens:
It behind the lens, its upright, magnified, virtual.
- The image is as far behind the mirror as the object in front.
- The image is NOT upside down.
- The image is the same size as the object.
- The image is laterally inverted, which means left and right are swapped around.
- angle of incidence i = angle of reflection r
- The image only can be seen by the eye. It cannot be focused onto a screen, which is why it is virtual.
General wave properties
- What are waves:
1) Wave transfer energy without transferring matter.
- How to describe a wave:
speed, frequency, wavelength and amplitude
1) Wavelength(sound: lambda): the distance from one peak of the wave to the next (unit: m)
2) Amplitude(A): the distance from the centre of a vibration to the peak (unit: m)
3) Frequency(f): the number of complete wave passing a point every second (unit: Hz)
4) Speed(v): the distance of a wave travelled per second (unit: m/s)
Formula: Velocity(m/s) = frequency(Hz) x wavelength(m)
Speed of sound in the air is 330 m/s
Speed of light in the vacuum is 3.0 x 10^8 m/s
- Transverse vs Longitudinal waves
1) Longitudinal waves:
In a longitudinal wave, the particles vibrate parallel to the direction of wave.
This leads to a series of compressions and rarefactions.
In compressions, the particles are closer together than normal. In rarefaction,
the particles are farther apart than normal.
A typical example of longitudinal wave is sound wave in the air
2) Transverse waves:
In a transverse wave, the particles vibrate perpendicular to the direction of wave.
This leads to a series of peak and troughs.
At peaks, the particles are displaced higher than normal, at troughs, they are displaced
lower than normal.
An example of a transverse waves is light.
- What is wavefront:
1)Wavefront is a surface containing points affected in the same way by a wave at a
2)Wavefronts can be represented as lines which are always perpendicular to the
direction of wave travel. The distance between one wavefront and the next is one
1) Waves reflect at a surface. There is no change in frequency, speed or wavelength on reflection
angle of incidence i = angle of reflection r
1) Wave changes its direction of propagation because of the different speed in two mediums.
2) Water waves travel faster in the deep water than in shallow water.
3) Band to the normal with a shorter wavelength.
4) Frequency no change.
2)Sound waves are longitudinal, producing a series of compressions and rarefactions in the medium.
1) The faster vibrations, the higher the frequency and hence the higher the pitch of the vibrations
2) The greater the amplitude of the vibrations, the louder the sound.
- Same high speed in vacuum.
- Safety concerns of the e.m. radiation: keep distance.