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Multiple choice image formation by lens image formation by lenses ray optics and optical instruments optics physics

When the reflecting or refracting rays do not actually intersect but appear to intersect when produced backwards, 

  1. a real image is formed

  2. a virtual image is formed

  3. either a real or a virtual image is formed

  4. neither a real nor a virtual image is formed
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

A virtual image is formed when the refracting or reflecting rays do not actually intersect but appear to intersect when they are produced backwards.

Multiple choice image formation by lens image formation by lenses ray optics and optical instruments optics physics

Comparing real and virtual images, we may say that

  1. real images can not be obtained on a screen and virtual images can be

  2. virtual images can not be obtained on a screen and real images can be

  3. both real and virtual images can be obtained on a screen

  4. neither real nor virtual images can be obtained on a screen
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Real images can be obtained on a screen. Virtual images can not be obtained on a screen.

Multiple choice image formation by lens image formation by lenses ray optics and optical instruments optics physics

A real image is formed by

  1. actual intersection of the refracting or reflecting rays

  2. imaginary intersection of the refracting or reflecting rays produced backwards

  3. either actual or imaginary intersection of the reflecting or refracting rays

  4. neither actual nor imaginary intersection of the reflecting or refracting rays
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

When the refracting or reflecting rays actually intersect, a real image is formed.

Multiple choice image formation by lens image formation by lenses ray optics and optical instruments optics physics

A thin convex lens of focal length $30.00\ cm$ forms an image $2.00\ cm$ high, of an object at infinity. A thin concave lens of focal length $20.00\ cm$ is placed $26.00\ cm$ from the convex lens on the side of the image. The height of the image now is

  1. $1.00\ cm$
  2. $1.25\ cm$
  3. $2.00\ cm$
  4. $2.50\ cm$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

The convex lens forms an image at its focal point (30 cm). The concave lens is placed 26 cm from the convex lens, so the image acts as a virtual object for the concave lens at a distance of 4 cm (30 - 26 = 4 cm). Using the lens formula 1/f = 1/v - 1/u, where f = -20 and u = +4, we get 1/v = 1/-20 + 1/4 = 4/20, so v = 5 cm. Magnification m = v/u = 5/4 = 1.25. Final height = 1.25 * 2 cm = 2.50 cm.

Multiple choice image formation by lens image formation by lenses ray optics and optical instruments optics physics

When the distance between the object and the screen is more than 4f, we can obtain the image of the object on the screen for the two positions of the lens. It is called displacement method.In one case, the image is magnified. If $I _1$ and $I _2$ be the sizes of the two images, then the size of the object is

  1. $(I _1+I _2)/2$
  2. $I _1-I _2$
  3. $\sqrt{I _1\,I _2}$
  4. $\sqrt{I _1/I _2}$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

In the displacement method, the object size O is related to the two image sizes I1 and I2 by the geometric mean formula O = sqrt(I1 * I2). This result is derived from the magnification formulas m1 = I1/O = v/u and m2 = I2/O = u/v.

Multiple choice image formation by lens image formation by lenses ray optics and optical instruments optics physics

If $I _1$ and $I _2$ be the size of the images respectively for the two positions of lens in the displacement method, then the size of the object is given by

  1. $I _1/I _2$
  2. $I _1\times I _2$
  3. $\sqrt{I _1\times I _2}$
  4. $\sqrt{I _1/I _2}$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Similar to the previous question, the size of the object in the displacement method is the geometric mean of the two image sizes, O = sqrt(I1 * I2).