Theme 3B: Optics
Reflection
Refraction
Total internal reflection
Electromagnetic Spectrum
Uses of Lenses
Lenses
Angle of incidence = angle of reflection provided that the incident ray, the normal and the reflected ray all lie on the same plane.
Characteristics of an image
Same size as object
Equidistant from the mirror as object
Laterally inverted
Virtualc
Upright
Air to Glass
Glass to Air
Ray is slowed down; bends towards the normal
Ray speeds up; bends away from the normal
Incident ray hits a semicircular glass block at the critical angle (glass = 41.8) and is refracted perpendicular to the normal (90 degrees).
Incident ray hits a semicircular glass block at an incident angle more than 41.8 and is reflected back not refracted.
Why semicircular?
Normal is perpendicular to tangent on the semicircular glass block
Incident ray travels on normal and so no refraction occurs.
Used in fibre optic cables
Used in binoculars/periscopes
Refractive index (optical density)
speed of light in air/speed of light in medium
real depth/apparent depth
Object at infinity = Image at F
Object beyond 2F = Image between F and 2F
Object at 2F = Image at 2F
Object between F and 2F = Image Beyond 2F
Object at F = Image at Infinity
Object between Lens and F = Image on same side as the object
Convex lens - converging lens
Concave lens - diverging lens
Virtual, upright, magnified
Real, inverted, diminished
Real, inverted, same size
Real, diminshed, inverted
Real, inverted, diminished
Gamma
X-Rays
Ultraviolet
Visible
Infrared
Microwaves
Radiowaves
Use: communication
Danger: none
Use: communication/cooking
Danger: can kill cells due to heating up water
Use: Heat/remote control
Danger: absorbed by skin
Use: tanning/checking money
Danger: high doses can kill cells
ROYGBIV
Use: checking for broken bones
Danger: High doses can kill cells
Use: sterilisation
Danger: High doses can kill cells
Dispersion
When white light passes through a prism, it is refracted
It is dispersed into different colours due to different speeds
Red is deviated the least
Violet is deviated the most
Image on same side of object: magnifying glass
Image at F: burning a hole with a magnifying glass
Image between F and 2F: lens of a camera, human eye, objective lens of a refracting telescope
Image at 2F: inverting lens of a field telescope
Image beyond 2F: objective lens in a microscope
Image at infinity: searchlights
Magnification = Height of image/Height of object
Magnification = Image distance/object distance