Light Flashcards
What is specular reflection?
All the incident ray is reflected at the same angle
What is a plane mirror?
A flat mirror
What is diffuse reflection?
All the incident ray is reflected at lots of different angles
What is the angle of incidence equal to?
The angle of reflection
Give four characteristics of a reflected image
Objects are the same size
Same distance away from the mirror
The image is virtual
Laterally inverted - left and right sides are swapped.
What is refraction?
The bending (changing direction) of a light ray as it travels from one transparent substance to another one with different optical density
What happens when a ray of light enters an optically denser substance?
It bends towards the normal
What happens when a ray of light leaves an optically denser substance?
It bends away from the normal
Will a refracted ray ever bend beyond the normal?
No (so there is no bending when a ray crosses a boundary at a right angle)
What is the relationship between sin(i) sin(r) and the refractive index(n)
Sin(i) / sin(r) = n
If the two boundaries of the substance are parallel what will the rays be?
Parallel
What is Snell’s Law?
When an incident ray passes into a material: n = sin(i) / sin(r)
When you divide sin(i) by sin(r) you get the same number (for a particular transparent substance)
What is Snell’s Law?
When an incident ray passes into a material: n = sin(i) / sin(r)
When you divide sin(i) by sin(r) you get the same number (for a particular transparent substance) - the ratio is constant
Describe what happens to the light beam inside the prism (right angled triangle entering on the left)
It is totally internally reflected off the longest face (angle of incidence >or equal to>critical angle)
What happens when a ray of white light passes through a prism?
The white light emerges as a band of colours called a spectrum. This is caused because light is a mixture of colours and each colour travels through the prism at a slightly different speed, so each colour is refracted at a different angle. The prism has a different refractive index for each colour. This process is called dispersion.
Describe what happens to the light beam inside the prism (right angled triangle entering at the base)
It is totally internally reflected off the two short faces (angle of incidence>or equal to>critical angle
What is total internal reflection used in?
Prisms in periscopes, binoculars and bicycle reflectors, optic fibres
What is a virtual image?
A virtual image is when the rays are diverging so the light from the object APPEARS to be coming from a completely different place. E.g. A mirror
When does total internal reflection happen?
When you go from a more optically dense object to a less optically dense object and when the angle of incidence is greater than the critical angle
Is looking through a magnifying lens real or virtual?
Virtual because the image looks bigger than it actually is.
3 things you need to say to describe an image properly:
- How big it is compared to the object.
- Whether it’s upright or inverted (upside down) relative to the object.
- Whether it’s real or virtual.
What is a luminous object?
Objects that emit their own light.
What are non-luminous objects?
Objects that just reflect light, this is the case for most objects.
How do optical fibres actually work?
Optical fibres are very thin strands made from two types of glass. The optically less dense glass surrounds an optically more dense glass. As the fibres are very narrow, light always strikes the boundary of the two glasses at an angle greater than the critical angle. All the light is reflected so no light escapes across the boundary, creating a ‘light pipe’. providing a path the light follows even when the fibre is curved
What is meant by the term total internal reflection?
At an angle of incidence greater than the critical angle, all light is reflected back, and there is no more refraction. The reflection occurs inside the more optically dense medium
What is the critical angle (c) ?
The angle of incidence at which total internal reflection begins
What is the refractive index of a material?
The ratio of the sine of the angle of refraction and the sine of the angle of incidence.
It tells you how fast light is travelling in a material
Describe an experiment to find the refractive index of glass
1) 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
2) You need to draw in the normal at 90° to the edge of the block, at the point where the ray enters the lock
3) Use a protractor to measure the angle of incidence (i) and the angle of refraction (r). Calculate the refractive index. Repeat to make the experiment reliable.
What is the speed of light?
300,000,000 m/s
How does the prismatic periscope work?
Light passes normally through the surface AB of the first prism (it enters the prism at 90°) and so is undeviated.
It then strikes the surface AC (hypotenuse) of the triangular prism at an angle of 45°.
The critical angle for glass is 42° so the ray is totally internally reflected and turned through 90°.
On emerging from the first prism the light travels to a second prism which is positioned such that the ray is again totally internally reflected.
The ray emerges parallel to the direction in which it was originally travelling.
What are the advantages of sending signals using optic fibres?
They are less prone to noise/heating
They can send more information/data per second
