Question

1) the current in a 100w, 220v electric light is ?

2) in a red light a lens has a focal length 25cm. in blue light the probable focal length will be----

a) 25.5 b) 24.5 c) 15 d) 35

Answer 1

QUESTION:

1) the current in a 100w, 220v electric light is ?

2) in a red light a lens has a focal length $25\mathrm{cm}$. in blue light the probable focal length will be---

a) 25.5 b) 24.5 c) 15 d) 35

SOLUTION

1. According to the Joule's law:

$$\begin{array}{l} P = I U \\ I = \frac{P}{U} \\ I = 0.4545 \text{ A} = 454.5 \text{ mA} \end{array}$$

2. The relation between optical power, focal length and refractive index is:

$$D = \frac{1}{f} = (n - 1) \left(\frac{1}{R_1} - \frac{1}{R_2}\right)$$

Because of dispersion the index of refractivity of glass is slightly greater for a blue light than for a red light (for visible light, refraction indices $n$ of glass decrease with increasing wavelength $\lambda$). Hence for a blue light focal length is slightly smaller and probably is $24.5~\mathrm{cm}$.

ANSWER

1. 454.5 mA

2. b) 24.5 cm