Download Reflection,Refraction, Lenses

Physics REVISION – Light - Reflection
The law of reflection
Sound waves and light waves reflect from surfaces. The angle of
incidence equals the angle of reflection. This is called the Law of
Reflection. So, if a wave hits a mirror at an angle of 36°, it will be
reflected at the same angle (36°).You can investigate the law of
reflection using a light box, mirror and angle measurer.
The plane mirror and constructing a ray diagram
Ray diagrams are drawn to explain reflection in a plane mirror (a flat
mirror). Light waves reflect from surfaces. When waves reflect, they
obey the law of reflection: the angle of incidence equals the angle of
reflection.
In a ray diagram, the mirror is drawn as a straight line with thick
hatchings to show which side has the reflective coating. The light rays
are drawn as solid straight lines, each with an arrowhead to show the
direction of travel.
Light rays that appear to come from behind the mirror are shown as
dashed straight lines. The incident rays (the solid lines) should obey
the law of reflection: the angle of incidence equals the angle of
reflection.
The image in a plane mirror is:
• virtual (it cannot be touched or projected onto a screen)
• upright (if you stand in front of a mirror, you look the right way up)
• laterally inverted (if you stand in front of a mirror, your left side
seems to be on the right in the reflection).
Physics REVISION – Refraction and
Refraction
Sound waves and light waves change speed when they pass across the boundary
between two substances or mediums with different densities, such as air and glass.
This causes them to change direction and this effect is called refraction.
Refraction doesn't happen if the waves cross the boundary at an angle of 90°
(called the normal) - in this case, they carry straight on.
The refraction follows a regular patterns:
When a wave passes from a less dense medium to a more dense medium such as air
to glass the ray slows down and moves towards the normal line. On the other hand
as the wave moves from a more dense medium to a less dense medium such as
water to air it speeds up and moves away from the normal and leaves the medium at
the same angle it enters the medium. The angle of refraction is proportional to the
angle of incident. If one goes down so does the other! The angle of refraction is
normally smaller than the angle of incident.
Critical angle
Due to refraction waves going from a dense medium to a less dense medium speed
up at the boundary between them. This causes light rays to bend when they pass
from glass to air at an angle other than 90°. Beyond a certain angle, called the critical
angle, all the waves reflect back into the glass. We say that they are totally internally
reflected. All light waves, which hit the surface beyond this critical angle, are
effectively trapped. The critical angle for most glass is about 42°.
The critical angle can be calculated as:
sin c = 1 / n
Where:
c is the critical angle and n is the refractive index
Example - Calculate the critical angle for red light incident ray on
a water-air interface. The refractive index of water is 1.33 for this
colour of light.
Critical angle of water for this light = 48.8°
Physics REVISION – Refraction and
The inside surface of water or glass can act like
a perfect mirror, depending on the angle at
which light strikes it. When an angle of
incidence greater than the critical angle, the
light cannot pass through the surface - it is all
reflected. This is called total internal
reflection.
Refractive index
The speed of light is determined by the material (or
medium) through which it travels. As it leaves one
medium and enters another, it changes speed and so
refracts. The refractive index is the extent to which light
is refracted and a measure in the change of speed when
it passes from one medium to another. It is calculated
as:
Equation: n = sin i/ sin r
where:
n is the refractive index of the medium
i is the angle of incidence
r is the angle of refraction
A refractive index has no units. Air has an index of 1.0,
water is 1.3 and many types of glass are around 1.5. This
means that light travels 1.3 times as fast in air than it
does in water, and 1.5 times as fast in air than in glass.
Use a rectangular glass block and an angle measurer to
determine the angles of incidence and refraction as light
rays pass from air into the block. Once you have
determined the two angles you can calculate the
refractive index using the equation above.
Worked example
A beam of light hits a glass block. The angle of
incidence is 55°. The angle of refraction inside
the block is 33°. Calculate the refractive index.
1. Work out the sine of angle i
sin 55 = 0.819
2. Work out the sine of angle r
sin 33 = 0.545
3. Divide sin i by sin r
refractive index = sin i ÷ sin r
refractive index = 0.819 ÷ 0.545 = 1.50
An optical fibre is a thin rod of high-quality
glass or transparent plastic.
Light goes in at one end undergoes repeated
total internal reflection, even when the fibre
is bent, and emerges at the other end. Very
little light is absorbed by the glass/plastic so
it comes out the other end as bright as it
goes in, even if it is several km long
Optical fibres can carry telephone calls and
internet data. The information is coded and
sent along as pulses of light.
A bundle of optical fibres are used in
endoscopes that allow surgeons to see inside
their patients.
Optical fibres can also carry enormous
amounts of information as pulses of light.
Physics REVISION – Lenses and Image creation
Lenses REFRACT light and are usually used to form IMAGES. There
are 2 types of lenses Convex and Concave. You only need to know
about concave lenses. Convex lenses converge light rays to a focal
point or principal focus behind the lens. The distance between the
principal focus and the lens is called the focal length.
convex concave
In practice, light is refracted
at both surfaces of the lens
but for simplicity we draw
only one refraction as if it
happened at the centre line.
All ray diagrams are drawn in
PENCIL and use a RULER!
Physics REVISION – Dispersion of light
A prism makes white light disperse,
forming a spectrum.
Refraction of light in a 60-60-60 prism can
be seen below.
What we can see when dispersion happens:
White light is a mixture of colours – together
they make white light.
Light waves or other types of radiation
containing only one colour have a single
wavelength or frequency and are known as
Monochromatic.
Red light refracts the least (long wavelength.)
Voilet light refracts the most (short
wavelength.)