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Question
A concave mirror has a focal length of 20 cm. Find the position or positions of an object for which the image-size is double of the object-size.
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Solution
Using sign conventions, given,
Focal length of the concave mirror:
f = −20 cm
As per the question,
Magnification (m) is:
\[m = - \frac{v}{u} = 2\]
⇒ v = −2u
Case I (Virtual image):
Using mirror formula,
\[\frac{1}{v} + \frac{1}{u} = \frac{1}{f}\]
\[ \Rightarrow - \frac{1}{2u} - \frac{1}{u} = \frac{1}{f}\]
\[ \Rightarrow \frac{3}{2u} = \frac{1}{f}\]
\[ \Rightarrow u = \frac{3f}{2} = 30 \text{ cm }\]
Hence, the required positions of objects are 10 cm or 30 cm from the concave mirror.
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To a distant observer, the light appears to be coming from somewhere below the ground. The observer naturally assumes that light is being reflected from the ground, say, by a pool of water near the tall object. Such inverted images of distant tall objects cause an optical illusion to the observer. This phenomenon is called mirage. This type of mirage is especially common in hot deserts. Based on the above facts, answer the following question : |
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The sky would appear red instead of blue if
