Light Flashcards
What type of waves are light waves?
light waves are transverse waves
What three things can light waves be?
reflected
refracted
diffracted
What allows us to see most objects?
reflection of visible light is what allows us to see most objects
light bounced off the objects into our eyes
What happens when light reflects from an uneven surface?
when light reflects from an uneven surface such as a peice of paper, the light reflects off at all different angles and you get a diffuse reflection
What happens when light reflects from an even surface?
when light reflects from an even surface (smooth and shiny like a mirror) then it’s all reflected at the same angle and you get a clear reflection
What is the law of reflection?
the angle of incidence = the angle of reflection
When are images formed?
images are formed when light waves cross
Always measure from the … to the ray itself
Always measure from the Normal to the ray itself
When is a real image produced?
a real image is produced when real rays of light cross
When are virtual images produced?
a virtual image is produced when virtual rays of light cross
What are the differences between a real image and a virtual image
a real image is produced when real rays of light cross and can be projected
a virtual image is produced when virtual rays of light cross and cannot be projected
What is three things is virtual image compared to the real image?
the virtual image is:
the same size
laterally inverted
the same distance behind the mirror as the real object is in front
What are our brains hard wired to think?
our brains are hardwired to think that light travels in straight lines
we can be fooled to think the image behind the mirror because our brains do not know that the real rays of light have been reflected of the mirror and have changed direction
How do you construct ray diagram to illustrate the formation of a virtual image in a plane mirror?
- Draw the virtual image. The image is the same size as the real image and is the same distance behind the mirror as the real object is in front
- Draw a reflected ray going from the top of the virtual image to th top of your eye. Draw a bold line for the part of the ray between the mirror and eye, and a dotted line for the part of the ray between the mirror and the virtual image
- Draw an incident ray going from the top of the onject to the mirror. The incident and reflected rays follow the law of reflection - but you don’t have to measure any angles. Draw the ray from the object to the point where the reflected ray meets the mirror
- Now you have an incident ray and a reflected ray for the top of the image. Repeat steps 2 and 3 again for the bottom of the image - a reflected ray going from the image to the bottom of the eye, then an incident ray from the object to the mirror
The normal is always …o (parallel/perpendicular) to the mirror
The normal is always 90o (perpendicular) to the mirror
What is refraction?
refraction is the bending of light which occurs at the boundary of two different medium, due to a change of density which changes the speed of light - it occurs when light travels from one medium to another
‘changes direction’
EM waves, therefore light waves, usually travel faster/slower in denser media
The light rays get closer together/farther apart
EM waves, therefore light waves, usually travel slower in denser media
The light rays get closer together
Design an experiment to investigate the refraction of light using a retangular block of a particular material (e.g. glass) resting on top of a piece of paper. What happens?
- Shine a light ray at an angle into the block. Some of the light is reflected, but a lot of it passes through the glass and gets refracted as it does so
- Trace the incident and emergent rays onto the piece of paper and remove the block. You can draw in the refracted ray through the block by joining the ends of the other two rays with a straight line
- You should see that as the light passes from the air into the block (a denser medium), it bends towards the normal. This is because it slows down
- When the light reaches the boundary on the other side of the block, it’s passing into a less dense medium so it speeds up and bends away from the normal (some of the light is also reflected at this boundary)
- The light ray that emerges on the other side of the block is now travelling in the same direction it was to begin with - it’s been refracted towards the normal and then back again by the same amount
- You can measure the angles between the rays and the normal to work out the refractive index of the material in the block
When light slows down, it bends towards/away from the Normal
When light speeds up, it bends towards/away from the Normal
When light slows down, it bends towards the Normal
When light speeds up, it bends away from the Normal
What happens when light travels through a block?
As the light passes from the air into the block (a denser medium), it bends towards the normal. This is because it slows down
When the light reaches the boundary on the other side of the block, it’s passing into a less dense medium so it speeds up and bends away from the normal
The light ray that emerges on the other side of the block is now travelling in the same direction it was to begin with - it’s been refracted towards the normal and then back again by the same amount
What is the refractive index and its equation?
the refractive index of a material (n) is the ration of the speed of light in free space (air) to the speed ofl ight in the material:
n = speed of light in a vaccum (c) ÷ speed on light in material (v)
n = c ÷ v
Light slows down/speeds up in glass, so the refractive index of glass is high/low
The refractive index of water is a bit higher/lower so the light doesn’t slow down/speed up as much in water as in glass
Light slows down a lot in glass, so the refractive index of glass is high
The refractive index of water is a bit lower so the light doesn’t slow down as much in water as in glass
What is Snell’s Law?
when an incident ray passes into a material: n = sin i ÷ sin r
A beam of light travels from air into water. The angle of incidence is 23o. Refractive index of water = 1.33. Calculate the angle of refraction to the nearest degree
n = sin i ÷ sin r
sin r = sin i ÷ n
sin 23o ÷ 1.33
=0.29…
r = sin-1 (0.29…)
= 17o
How do you find the refractive index of glass using a glass block?
- Draw around a rectangular glass block on a piece of paper and direct a ray of light through it at an angle. Trace the incident and emergent rays, remove the block, then draw in the refracted ray between them
- You then need to draw in the normal at 90o to the edge of the block, at the point where the ray enters the block (this is where you measure the ray of light)
- Use a protactor to measure the angle of incidence (i) and the angle of refraction (r)
- Calculate the refractive index (n) using Snell’s Law: n = sin i ÷ sin r
N.B. The refractive index should be around 1.5
What happens to the angle of refraction as you increase the angle of incidence?
as the angle of incidence increases, the angle of refraction gets close to 90o
Desrcibe an experiment to demostrate total internal reflection using a semicircular block
- Draw around a semicircular glass block on a piece of paper and aim an incident light ray at the curved edge of the block so that it always enters at right angles to the edge. Trace the incident and emergent rays, remove the block, then draw in the refracted ray between them
- You then need to draw in the normal at 90o to the edge of the block, at the point where the ray enters the block (this is where you measure the ray of light)
- Use a protactor to measure the angle of incidence (i) and the angle of refraction (r)
- Calculate the refractive index (n) using Snell’s Law: n = sin i ÷ sin r
What happens if the angle of incidence is less than the critical angle?
most of the light passes out but a little bit of it is internally reflected
What happens if the angle of incidence is equal than the critical angle?
the emerging ray comes out along the surface
there is quite a bit of internal reflection
What happens if the angle of incidence is greater than the critical angle?
no light comes out
it’s all internally reflected i.e. total internal reflection
What is total internal reflection?
complete reflection of a light ray reaching a boundary at an angle greater than the critical angle
See Experiment Write Ups for the Snell’s Law Experiment
What does the angle of incidence equal when the angle of refraction = 90o
when the angle of refraction = 90o the angle of incidence = the critical angle
What is the equation for finding the critical angle?
sin C = 1 ÷ n (the refractive index of the material)
The higher the refractive index, the higher/lower the critical angle
The higher the refractive index, the lower the critical angle
The critical angle is 42o. Calculate the refractive index
sin C = 1 ÷ n
n = 1 ÷ sin C
= 1 ÷ (sin C)
= 1 ÷ 0.66…
= 1.49 (2 d.p)
How do optic fibres use total internal reflection?
Information, in the form of pulses of light, is sent down bundles of optical fibres
optic fibres made of plastic or glass consist of a central core surrounded by cladding that has a lower refractive index
the core of the fibre is so narrow that light signals passing through it always hit the core-cladding boundary at angles higher than the critical angle - so the light is always totally internally reflected
it only stops working if the fibre is bent too sharply
How do you use total internal reflection in a periscope?
Two right angle prisms can be used to make a periscope.
At the back face of the prisms there is total internal reflection.
Please see the picture below.
Different wavelengths of light refract by the same/different amounts.
What does this do to white light as it enters a prism?
Different wavelengths of light refract by different amounts
White light disperses into different colours as it enters a prism
What is the relationship between wavelength and refraction?
The shorter the wavelength of the light, the more it is refracted
Which colour is refracted the most? Why?
violet light is refracted the most because it has the shortest wavelength
Which colour is refracted the least? Why?
red light is refracted the least as it has the longest wavelength
