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(i) What is meant by refraction of light? Write the laws of refraction.
(ii) If two lenses of focal length $+20\ \text{cm}$ and $-10\ \text{cm}$ are kept in contact, find out the net power of the lens combination. Will the combination behave as converging lens or a diverging lens ?
(ii) If two lenses of focal length $+20\ \text{cm}$ and $-10\ \text{cm}$ are kept in contact, find out the net power of the lens combination. Will the combination behave as converging lens or a diverging lens ?
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(i)
$\bullet$ When a ray of light travels obliquely from one transparent medium to another, the direction of propagation of light in the second medium changes. This phenomenon is known as refraction of light. ($\frac{1}{2}$ Mark)
$\bullet$ Laws of refraction of light
(1) The incident ray, the refracted ray and the normal to the interface of two transparent media at the point of incidence, all lie in the same plane. (1 Mark)
(2) The ratio of sine of angle of incidence to the sine of angle of refraction is a constant, for the light of a given colour and for the given pair of media. (1 Mark)
(ii)
$\text{P} = \frac{1}{\text{f}}$ ($\frac{1}{2}$ Mark)
$\text{P}_1 = \frac{1}{20 \text{ cm}} = \frac{100}{20 \text{ m}}$
$\text{P}_1 = +5 \text{ D}$ ($\frac{1}{2}$ Mark)
$\text{P}_2 = \frac{1}{-10 \text{ cm}} = \frac{-100}{10 \text{ m}}$
$\text{P}_2 = -10 \text{ D}$ ($\frac{1}{2}$ Mark)
Net Power of the lens combination = $\text{P} = \text{P}_1 + \text{P}_2$
$\text{P} = +5 \text{ D} - 10 \text{ D}$
$\text{P} = -5 \text{ D}$ ($\frac{1}{2}$ Mark)
The combination will behave as a diverging lens (concave lens). ($\frac{1}{2}$ Mark)
$\bullet$ When a ray of light travels obliquely from one transparent medium to another, the direction of propagation of light in the second medium changes. This phenomenon is known as refraction of light. ($\frac{1}{2}$ Mark)
$\bullet$ Laws of refraction of light
(1) The incident ray, the refracted ray and the normal to the interface of two transparent media at the point of incidence, all lie in the same plane. (1 Mark)
(2) The ratio of sine of angle of incidence to the sine of angle of refraction is a constant, for the light of a given colour and for the given pair of media. (1 Mark)
(ii)
$\text{P} = \frac{1}{\text{f}}$ ($\frac{1}{2}$ Mark)
$\text{P}_1 = \frac{1}{20 \text{ cm}} = \frac{100}{20 \text{ m}}$
$\text{P}_1 = +5 \text{ D}$ ($\frac{1}{2}$ Mark)
$\text{P}_2 = \frac{1}{-10 \text{ cm}} = \frac{-100}{10 \text{ m}}$
$\text{P}_2 = -10 \text{ D}$ ($\frac{1}{2}$ Mark)
Net Power of the lens combination = $\text{P} = \text{P}_1 + \text{P}_2$
$\text{P} = +5 \text{ D} - 10 \text{ D}$
$\text{P} = -5 \text{ D}$ ($\frac{1}{2}$ Mark)
The combination will behave as a diverging lens (concave lens). ($\frac{1}{2}$ Mark)