Light reflection and refraction Worksheet-17
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The refractive index of glass is 3/2 and refractive index of water is 4/3. What would be the refractive index of water with respect to glass?
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What are the conditions for no refraction of light ?
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A ray of light passing through centre of curvature of a spherical mirror retraces its path on reflection from the mirror.
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A concave mirror is used as a head mirror by ENT specialists. The same mirror can also be used as a shaving mirror. Why?
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State mirror formula. How does f change when object distance u from the mirror is changed ?
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The formula for linear magnification of a spherical mirror is What determine the sign of m ? What is the significance of this sign?
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The image formed by a convex mirror of focal length 20 cm is a quarter of the object What is the distance of the object from the mirror?
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How much time will light take to cross 2 mm thick glass pane if refractive index of glass is 3/2 ?
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How do you find the rough focal length of a convex lens? Is the same method applicable to a concave lens ?
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Which factors determine the focal length of a lens ?
Answer:
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Refractive index of water w.r.t. glass
∴ light travels slower in glass than in water.
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Refraction of light shall not occur when
(i) Light is incident normally on a boundary of two media.
(h) Boundary that separates the two media which have equal refractive indices.
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This is because normal to spherical mirror at any point is the line joining that point to the centre of curvature of the mirror. Therefore, any ray passing through C falls normally on the mirror. It would retrace its path on reflection from the mirror.
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A concave mirror converges the rays of light falling on it in a direction parallel to the principal axis, on to its focus. That is why ENT specialists use it as a head mirror.
When an object is held between the pole and principal focus of a concave mirror, the mirror forms a virtual, erect and magnified image of the object. That is why concave mirror is used as a shaving mirror also.
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The mirror formula is , where the symbols have standard meaning.
When u is changed, v changes, but f remains constant. This is because focal length of mirror depends only on radius of curvature of the mirror.
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In the formula, the sign of m is determined by the signs of h1 and h2. When m is positive, the image is virtual and erect and when m is negative, the image is real and inverted.
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Given f = 20 cm,
∴ v = –u/4
From
or ; u = –60 cm
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Here, t = ?, x = 2 mm = 2 × 10–3 m,
From m/s
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The rough focal length of a convex lens is obtained by forming sharp image of a very distant object on a screen. The distance of the screen from the lens gives the rough focal length of the lens. This method will not be applicable to a concave lens, as image formed by a concave lens is virtual so it can not be taken on a screen.
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The focal length of a lens depends on (i) radii of curvature of the surfaces of the lens, (ii) nature of material of the lens, (iii) nature of medium in which lens is placed.