P14 Flashcards
Light
What is the law of reflection ?
angle of incidence = angle of reflection
What is specular reflection ?
when light rays are reflected off of a smooth, flat surface, producing a mirror image
[] the rays reflect all at the same angle and parallel to one another
What is diffuse reflection ?
when light rays are reflected off of a rough or bumpy surface, producing no mirror image but instead an image of the object itself
[] rays reflect at many different angles and are not parallel to one another
What is a virtual image ?
- an image that is formed after the light rays have already been reflected instead of converging directly
[] your reflection in a mirror is a virtual image, as the light had reflected on the mirror before converging on the retina of your eye - virtual images cannot be projected onto a screen
- virtual images seem to come from “behind” the mirror/lens etc.
What is a real image ?
- an image that can be projected onto a screen
- image formed where the light rays converge
What is refraction ?
the change in direction of waves as they cross a boundary from one medium to another, both with different densities
- the angle of refraction in relation to the angle of incidence:
[] less -> more dense medium, angle is smaller (wavefront towards normal)
[] more -> less dense medium, angle is larger (wavefront away from normal)
Describe how you can investigate refraction of light
- gather equipment
[] ray box
[] glass block
[] paper
[] pencil
[] protractor - set glass block on page and trace it with the pencil
- turn off lights and turn on ray box so that the ray hits the block at an angle and refracts
- mark the points where the ray enters and exits the block
- trace over the rays outside of the block with a ruler
- turn off the ray box and turn on the lights
- connect the entrance and exit points with a ruler
- measure the angles of incidence and refraction
Describe how people can see colour
- white light (sunlight) contains all colours of the rainbow
- certain pigments reflect only certain wavelengths, absorbing the others
[] eg. red pigment reflects only red light wavelengths and absorbs all other colours - these reflected colour lights converge in the retina of the eye and form a red image
How do colour filters work ?
Absorb all colour wavelengths from light and transmit only those the same colour(s) of the filter
- eg. purple filter would transmit both red and blue light waves, but no others
Describe how to investigate reflection and refraction of light rays from a ray box on different (coloured/smoothness of) surfaces
- set glass block on page and trace it with the pencil
- using a protractor and a ruler, draw a normal perpendicular to one of the long edges of the block’s outline
- place the block back on its outline
- turn off lights and turn on ray box with a slit to CREATE A NARROW BEAM OF LIGHT so that the ray hits the block at the point where the normal meets the block and refracts as well as reflects
- mark the path of the reflected ray
- mark the points where the ray enters and exits the block
- trace over the rays outside of the block with a ruler
- turn off the ray box and turn on the lights
- connect the entrance and exit points with a ruler
- measure the angles of incidence and refraction, as well as of incidence and reflection
- repeat with a block of a different material with the incident ray at the same angle as before, like perspex, and then again with another
- should find that angles of reflection do not change as it doesn’t depend on the block’s material, but angles of refraction depend on the material
- when investigating reflection on a coloured surface vs a plane mirror, should find that only the colour light of the surface is reflected, and with the plane mirror, the white light in its entirety is reflected
How does light interact with transparent objects ?
- transmit ALL incident light
- absorb no incident light
- transparent objects give a clear image of the things on the other side due to this
How does light interact with translucent (semi-transparent) objects ?
- light is transmitted through the object, BUT the light is scattered or refracted internally due to many different internal boundaries where the waves may cross
- give a blurry image of the other side, but can still see colours etc.
How does light interact with opaque objects ?
opaque objects absorb almost all incident light if coloured (reflecting the corresponding wavelength of coloured light), or reflect all incident light if white, so you can’t see through an opaque object at all
What is a convex lens and how does it refract light rays ?
- lens bulges outwards in the middle
- converges parallel incident rays to the principle focus (the point where the rays meet)
- animal eyes and cameras use convex lenses
- when object is outside the focal length of the lens, real image formed
- when object is inside the focal length of the lens, virtual image formed
What is a concave lens and how does it refract light rays ?
- lens curves inwards at its middle
- diverges parallel incident rays
What is the focal length ?
distance from the centre of the lens to the principle focus
- depends on lens’ strength
Give the equation for magnification
magnification = image height/real height
Describe the image produced by a convex lens when the object is outside of the focal length of the lens (2 focal lengths away)
- inverted
- diminished
- real
Describe the image produced by a convex lens when the object is outside of the focal length of the lens (between 1 and 2 focal lengths away)
- inverted
- magnified
- real
Describe the image produced by a convex lens when the object is within the focal length of the lens
- same way up as object
- magnified
- virtual (have to trace rays back with dotted lines until they meet to show the image)
Explain how magnifying glasses work
- object placed within the focal length of the convex magnifying glass lens
- gets magnified
Describe the image produced by a concave lens
- diminished
- right way up
- virtual
