## Ray optics

Ray optics is based on the assumption that light travels along straight lines. Taking average wavelength of visible light λ = 6000Å=6×10^{7} m and diameter of pupil of human eye, a = 1mm=10-3m, the minimum distance a beam of light travels before its deviation from straight line path becomes significant is:

**Z _{F}=a^{2}/ λ=(10^{-3})^{2}/6×10^{-7}=1.67m.**

Z

_{F}is called Fresnel distance. In day to day work, the distance involved is smaller than the Frensel distance. Therefore, we ignore deviation from straight line path i.e. we assume that light travels along straight lines. This is ray optics.

### Source of light

There are three types of source of light Thermal sources, Gas discharge sources and luminescent sources.

### Photometry

Photometry is a branch of Ray optics, which deals with the measurement of light energy. In this connection, the following four terms are to be clearly understood.

1) Luminous flux (φ) of a source of light = amount of visible light energy emitted per second from the source.

2) Luminous Intensity (I) of a light source= luminous flux emitted per unit solid angle in any direction.

3) Illuminance (E) of a surface is the luminous reflected into our eyes from unit area of the surface.

### Efficiency of an electric light source

Efficiency of an electric light source = luminous flux/electric power. It is measured in lumen/watt.

### Lambart’s cosine law

According to this law E ∝ cos θ, where θ is the angle which the incident light makes with normal to the surface at that point.

### Photometer

A Photometer is used for comparing the luminous intensities of two sources. It is based on the property of human eye that it can detect whether two given surfaces are equally bright, E1=E2

### Velocity of light

Velocity of light in vacuum( c ) is an absolute constant, which has several applications in Physics. Roamer was the first to design an astronomical method for the determination of velocity of light. In the laboratory, velocity of light in air was measured first by Foucault using a rotating mirror. Fizeau developed a rotating wheel method for the determination of velocity of light in air. The most accurate method was the one given by Michelson.

In each method, C = distance traveled/time

taken In Foucault’s method, c = 4πnd/ θ, Where d is distance of rotating mirror from the concave mirror; n is frequency of the mirror θ is the angle turned by the mirror. In Fizeau’s method, c = 4mnd, Where m is number of teeth or spaces in the rotating wheel n is frequency of the wheel and d is distance between the mirror and the rotating wheel. In Michelson’s method, c = 16nd Where n is frequency of rotating mirror and d is distance between two large fixed concave mirrors. The most accurate value of c = 2.99797× 10^{8}m/s

### Reflection of light

Reflection of light is the phenomenon of change in the path of light without change in medium. Angle between incidence ray and normal is angle of incidence (i). Angle between reflected ray and normal is angle of reflection ( r ). According to the law of reflection ∠ i =∠ r.