The distance of an object from a spherical mirror is equal to the focal length of the mirror. Then the image:
A. must be at infinity
B. may be at infinity
C. may be at the focus
D. none
1 Answers
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A double convex lens (`R_(1)=R_(2)=10 cm`) having focal length equal to the focal length of a concave mirror. The radius of curvature of the concave m
Correct Answer - B (b) we have, `1/f=(1.5-1)(1/10+1/10)` `therefore` Focal length, f= 10 cm Radius of curvaature, R=2f=20 cm
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A short linear object of length b lies along the axis of a concave mirror or focal length f at a distance u from the pole of the mirror. The size of t
Correct Answer - B (b) Using mirror formula, `1/v+1/(-u)=1/(-f)` Multiplying with u, `u/v=(-u)/f)+1=(f-u)/f` `therefore` Magnification, m=`v/u=(f/(f-u))` Now, `abs(dv)=m^(2)abs(du)` Sizze of image=`(f/(f-u))^(2)`cdotb`
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An object placed at 20cm in front of a concave mirror produces three times magnified real image. What is the focal length of the concave mirror?
Correct Answer - A We know that, linear magnification, `m=(f)/(f-u)` Given object displaced, u=-20cm m=-3 (`therefore "all images aer inverted"`) `So, -3=(f)/(f-(-20))` `-3=(f)/(f+20) Rightarrow -3f-60=f` `4f=-60 Rightarrow f=-(60)/(4)=-15cm` Since, mirror is...
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A convex lens of focal length f forms an image which is 13/ times the size of the object. Then, the distance of object from the lends is
Correct Answer - A Given `f=-f Rightarrow v=(1)/(3)u` According to the lens formula, `(1)/(f)=(1)/(v)-(1)/(u)-(1)/(f)=(1)/((-1//2)u)+(1)/(u)-(1)/(f)` `(-3+1)/(u)-(1)/(f)=-(2)/(u) Rightarrow u=2f`
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A microscope is having objective of focal length and eye piece of focal length 6cm. If tube length 30cm and image is formed at the least distance of d
Correct Answer - B Given, length of tube, L=30cm, focal length of objective lens, `f_(v)=1cm focal length of eye piece, `f_(e)=6cm, 2D=25cm By compound microscope, `m=(L)/(f_(u))(1+(D)/(f_(e)))` `m=(30)/(1)(1+(25)/(6))=30xx((6+25))/(6)` `5xx31=155cm approx 150`
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A boy of height 1m stands in front of a convex mirror. His distance from the mirror is equal to its focal length. The height of his image is
Correct Answer - C Let f be foc al length of the convex mirror. According to new cartesiaan sign convention Object distance, ` u = - f`, focal length `= +...
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Assertion : The radius of curvature of a mirror is double of the focal length. Reason : A concave mirror of focal length f in air is used in a medium
Correct Answer - C Focal length of a mirror does not depend on the medium in which mirror is held. It depend on the radius of the sphere from which it...
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`{:("Column I","Column II"),((A)" An object is placed at a distance equal to focal length from pole before convex mirror",(p)" Magnification is "(inft
Correct Answer - (A)-q (B)-p (C)-s (D)-r m= `(f)/(f-u)` (A) Convex mirror u=-ve ,f=+ve `m = (f)/(f-(f)) = (1)/(2)` (B) Concave `rarr m = (-f)/(-f-(-f))= oo` (C) Concave mirror u= -...
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The image of an object placed in front of a concave mirror of focal length 15 cm is of the same size as the object.
Correct answer is (d) Zero At c, object produces same sized image. ∵ f = 15 cm, ∴ R = 30 cm object and image both are at c. therefore, distance between them is zero.
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The distance between an object and its doubly magnified image by a concave mirror is: [Assume `f`= focal length]
Correct Answer - A For `m=2` `m=(v)/(u)=2` `V=-2u` ………………..`(i)` `(1)/(f)=(1)/(v)+(1)/(u) rArr (1)/(f)=(1)/(-2u)+(1)/(u)` `rArr (1)/(f)=(1)/(2u) rArr u=(f)/(2)` `& v=-f` Distance between object `&` image `=f+f//2=3f//2` For `m=-2` `m=-(v)/(u)=-2` `v=2u` `rArr (1)/(f)=(1)/(2u)+(1)/(u) rArr...
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