Light
Light originates from the accelerated motion of electric charges. It consists of waves and forms part of the electromagnetic spectrum. It is a form of energy capable of stimulating the light-sensitive cells in the retina of the eye.
The amount of energy entering the eye is only a minute fraction of the energy emitted by the light source. They eye is only capable of receiving and focusing light only if the light entering our eye is nearly parallel.
Reflection
Laws:
1) The incident ray, the reflected ray and the normal all lie on the same plane
2) The angle of incidence is equal to the angle of reflection
Specular reflection: when light strikes a smooth surface
Diffuse reflection: when light strikes a rough surface
Characteristics of mirror image: same size as the object
object distance = image distance
virtual, upright, laterally inverted
Refraction
Laws:
1)The incident ray, the reflected ray and the normal all lie on the same plane
2) The ratio of sin i to sin r is constant (according to Snell’s Law)
Refractive index: n1 / n2 = speed of light in vacuum / speed of light in medium = sin i / sin r
When light enters into a denser medium, the refracted ray bends towards the normal. When light passes from a denser to a less dense medium, the refracted ray bends away from the normal.
Total internal reflection: If angle i > critical angle, the light ray cannot pass through the material and is total internally reflected at the boundary surface
Critical angle: angle i for which angle r in the optically less dense medium is 90
sin c = 1 / n
Optical Fibres
Flashes of light (from a laser) are used to send signals at very high speeds in optical fibres. Optical fibres have an inner glass core surrounded by an outer glass cladding, where the cladding has slightly lower refractive index than the core so that the light inside the core is always totally internally reflected
Advantages: Much lighter, thinner and cheaper
Transmit signals with very little loss over great distances
Free from electrical interferences
Carry large numbers of phone calls at any one time
Uses: View patient’s organs without surgery
Carry data for computers and TV programmes
Make telephone calls
Lenses
Thin converging lens refract incoming parallel light rays so that they converge to a point.
Focal length (f): Distance between the optical centre and the focal point
Linear magnification (M) = height of image / height of object
= distance of image from lens / distance of object from lens
= v / u
Ray Diagrams and Lens Equation
Thin lens equation to counter-check ray diagrams: 1/u + 1/v = 1/f
u = distance of object from lens
v = distance of image from lens
f = focal length of lens
