P14 Light Flashcards
What’s the range of wavelengths of visible light?
400nm - 700nm
When a wave refracts, what property is conserved out of wavelength, frequency and wave speed?
Frequency - directly proportional to energy (conservation of energy)
Besides refraction, what can happen to a wave at a boundary?
Reflection, transmission, absorption
describe the relationship between colour and frequency
- Every colour of visible light corresponds to a specific frequency
- Continuous spectrum
what is the function of lasers?
Produce light that is monochromatic and coherent
how do colour filters work?
- work by absorbing certain wavelengths
- and transmitting other wavelengths
Why do certain objects have certain colours?
- Certain wavelengths of visible light are reflected or transmitted and other wavelengths are absorbed
- The colours we see are the wavelengths that are reflected or transmitted
Why are red, blue and green the primary colours of light?
Because they can be mixed to produce other colours
What makes an object appear white or black?
Black - all wavelengths absorbed
White - all wavelengths reflected or transmitted equally
What is meant by opaque?
- Opaque objects don’t transmit any light
- They absorb scatter and/or reflect all wavelengths of visible light
What are pigments?
- Chemicals that absorb and reflect different wavelengths of visible light
- A red pigment will reflect wavelengths corresponding to red and absorb all other wavelengths
- Objects that appear white have no pigment; all wavelengths are reflect equally
What is meant by transparent and translucent?
- Transparent objects transmit all wavelengths of visible light, without any refraction
- Translucent objects scatter or refract light that passes through them, due to internal boundaries within the material that repeatedly change the direction of the light
What do waves transfer?
Energy
What causes a transverse wave?
Oscillations that are perpendicular to the direction of energy transfer
Describe how optical density affects the speed of the light wave travelling through it
The higher the optical density, the lower the speed of light through it
Imagine a light ray entering a less optically dense medium at 40 to the normal. How does the angle of refraction compare to the angle of incidence?
The angle of refraction is greater than the angle of incidence
How are wavelength and frequency affected when a wave is refracted?
- Frequency does not change
- Wavelength changes (either increases or decreases)
What happens when a light ray travels from a more optically dense material to a less optically dense material?
Light rays passing from a more dense to a less dense medium bend away from the normal. The angle of refraction is greater than the angle of incidence.
What happens when a light ray travels from a less optically dense material to a more optically dense material?
Light rays passing from a less dense to a more dense medium bend towards the normal. The angle of incidence is greater than the angle of refraction
Light path through convex (converging) lenses
When light rays enter a lens they are refracted as they enter and then again as they leave the lens
Drawing ray diagrams to show images formed by convex lenses
- Draw one ray from the top of the object, parallel to the principal axis which refracts through the principal focus
- Draw a ray from the top of the object directly through the centre of the lens (no refraction)
- Where the two rays meet is the top of the image
Description of the image formed by convex lenses
- Inverted
- Either smaller of magnified
- Real
- Close to the object, the image is virtual, magnified and upright
- Further than focal length from the object, the image is inverted, real and larger
- Beyond twice the focal length, the image is inverted, real and smaller
Concave (diverging) lens
- Concave lenses make parallel rays spread out (diverge)
- The rays seem to come from a ‘virtual focus’ on the other side
- Concave lenses are diverging lenses
- Used to correct short sight
Drawing ray diagrams to show images formed with concave lenses
- Draw a ray from the top of the object diagonally through the centre of the lens
- Draw one ray from the top of the object to the lens, parallel to the principal axis
- Draw a dotted line from the principal focus in front of the lens to the point where the ray parallel to the principal axis crosses the lens
- Extend that same ray beyond the lens with a solid arrow to show the ray diverging from the lens
- Where the two rays meet is the top of the image
Description of an image formed by concave lenses
- Always the right way up
- Always smaller
- Always produce a virtual image
How to calculate the magnification produced by a lens
magnification = image height / object height
What is the principal focus of a convex lens?
The place where all the rays hitting the lens parallel to the axis meet
What is meant by focal length?
The distance from the lens to the principal focus
What is a virtual image?
- A virtual image is an image formed by the eye/brain of the observer when the brain assumes that diverging rays entering the eye must have travelled in straight lines from a source.
- A virtual image is not real, so it cannot be picked up on a screen - the rays do not cross at the point in is formed at. Instead rays have to be traced back in a straight line to that point.
Explain the difference between a real and virtual image
- A real image of a point on an object is formed where rays of light converge at a point
- If a screen is placed at that point the image will appear on the screen
what are transparent objects?
- transmit all the incident light that enters the object
- no light is absorbed at the surface
- the transmitted light travels through the object
what are translucent objects?
- let light pass through them, but the light is scattered or refracted
- because there a lot of internal boundaries that change the direction of the light rays repeatedly
what is an opaque object?
- an object that absorbs all the light that reaches it
- the light is either reflected, scattered at the surface, or absorbed by the object
- no light travels all the way through the object
what is the difference between the image produced by a concave and convex lens?
the image produced by a convex lens can be either virtual or real, but the image produced by a concave lens is always virtual
what is a perfect black body?
- an object that absorbs all of the radiation incident on it
- a black body does not reflect or transmit any radiation
