Light Flashcards
3 main properties of light waves:
- Light is a transverse electromagnetic wave
- Travels at 3 x 10^8 in a vacuum
- Travels in straight lines as a ‘ray’ until it hits a surface
What is the normal?
Construction line which is drawn at 90 degrees to the surface where the ray hits
What does a periscope use?
2 mirrors tilted at 45 degrees
Rays turn through 90 degrees twice
2 mirrors mean any left-right inversion is cancelled out
What happens when light travels through a different transparent material
It travels slower as the material is denser
What happens when light enters a glass block
It bends towards the normal
What happens when light leaves a glass block
It bends away from the normal
What is refraction?
The bending of the direction of travel of a wave as it moves from substance into another
Investigating refraction practical:
1) place glass block on paper, draw around block
2) take a ray box and aim the box so that a single ray of light passes through the block
3) mark some points along the path of the ray
4) repeat the above process for different angles for the ray striking the block
What happens when waves go from deep to shallow water?
Wavelength decreases
Equation for refractive index
n = sin i / sin r
How to measure the refractive index?
1) gradient = rise / run
2) calculating sin i / sin r for several points and taking an average
Facts about refractive index:
- refractive index is a unit less quality
- a material with a greater refractive index will refract a ray of light more
- always greater than 1
- always give to 2dp
How is the critical angle shown?
Using a semi - circular block
No refraction as light enters glass as it passes along the normal, when it leaves the back face it refracts
What happens once the incident ray is passed 90 degrees in a semi-circular block
All of the light which hits the back face is reflected - total internal reflection
What are the 2 conditions needed for total internal reflection?
1) light must be going from a more dense medium towards a less dense one
2) the angle of incidence must be greater than the critical angle
Equation for critical angle and refractive index
Sin C = 1 / n
What happens when light is sent into an equilateral prism
Light separates into visible spectrum - dispersion
At the first face, colours refract by different amounts
At second face, colours refract again, away from normal as they re- enter air
What happens when white light enters into a right angled prism
If it enters the shorter sides along the normal - it doesn’t refract
If it hits a longer face, angle of incidence is 45 degrees (larger than critical angle) - total internal reflection occurs and ray is deviated by 90 degrees
What happens when 2 prisms are used in a periscope
Longer side along normal - doesn’t refract
Shorter side - total internal reflection occurs
When it hits other shorter side - undergoes TIR and ray is deviated by 180 degrees
Used in binoculars
What are optical fibres
Very thin tube of flexible glass
Narrow - total internal reflection occurs, any ray of light will bounce down the fibre without any light emerging from side of glass
Uses of optical fibres:
1) send signals at high speeds - internet/ communications links, use infra red pulses to send data
2) bundles of fibres can be used in endoscopes to see inside a human body
What is the critical angle
The angle of incidence that provides an angle of refraction of 90 degrees
What is total internal reflection
the complete reflection of a light ray reaching an interface with a less dense medium when the angle of incidence exceeds the critical angle.
