(a) What is a wavefront? (b) (i) State two practical uses of glass prisms. (ii) List two factors that determine the deviation of a ray of light travelling f...

(a) A wavefront is a surface or locus of points in a wave which are vibrating in the same phase.

(b)(i) Two practical uses of glass prisms are:

1. to change the direction of a beam of light, for example in binoculars or periscopes;
2. to disperse white light into its component colours, for example in a spectrometer.

(b)(ii) The deviation produced by a triangular glass prism depends on:

1. the angle of incidence of the ray;
2. the refractive index of the glass prism.

The refracting angle of the prism is also a factor.

(b)(iii) The graph of angle of deviation, d, against angle of incidence, i, is shown below. The minimum point is the point at which i = e.

(c)(i) Astronomical telescope in normal adjustment:

In normal adjustment, the real intermediate image formed by the objective lies at the first focal point of the eyepiece. The emergent rays are parallel and the final image is therefore at infinity.

(c)(ii)

For an astronomical telescope in normal adjustment,

\[M=\frac{f_o}{f_e}\]

Given \(M=5\) and \(f_o=100\text{ cm}\):

(α) Focal length of eyepiece

\[5=\frac{100}{f_e}\]

\[f_e=\frac{100}{5}=20\text{ cm}\]

(β) Length of telescope

\[L=f_o+f_e=100+20=120\text{ cm}\]