Class 10 Refraction through a Lens Mock Test (ICSE)

Topics in Refraction through a Lens

• Convex and concave lenses
• Principal rays and image formation
• Lens formula and magnification
• Power of a lens
• Magnifying glass

Refraction through a Lens practice questions with answers

1. Q1Multiple choice·1 mark
   Which of the following lenses always produces a virtual, erect, and diminished image for all positions of a real object?

   1. (a)Convex lens
   2. (b)Plano-convex lens
   3. (c)Double convex lens
   4. (d)Concave lens
   Show answer and step marks

   1. Correct option: (d) Concave lens. A concave lens diverges incident light rays and always produces a virtual, erect, and diminished image irrespective of the object's position.+1

   Answer: (d)
2. Q2Multiple choice·1 mark
   A concave lens has a focal length of $25\text{ cm}$. The power of this lens is:

   1. (a)-0.25 D
   2. (b)+0.25 D
   3. (c)-4.0 D
   4. (d)+4.0 D
   Show answer and step marks

   1. Correct option: (c) -4.0 D. Using $P = \frac{1}{f \text{ (in m)}}$ with $f = -0.25\text{ m}$ for a concave lens gives $P = \frac{1}{-0.25\text{ m}} = -4.0\text{ D}$.+1

   Answer: (c)
3. Q3Multiple choice·1 mark
   An object is placed at a distance of $20\text{ cm}$ in front of a convex lens of focal length $10\text{ cm}$. The nature and position of the image formed is:

   1. (a)Virtual, erect, at $20\text{ cm}$ on the same side
   2. (b)Virtual, erect, at $10\text{ cm}$ on the other side
   3. (c)Real, inverted, at infinity
   4. (d)Real, inverted, at $20\text{ cm}$ on the other side
   Show answer and step marks

   1. Correct option: (d) Real, inverted, at $20\text{ cm}$ on the other side. Using the lens formula $\frac{1}{v} - \frac{1}{u} = \frac{1}{f}$ with $u = -20\text{ cm}$ and $f = +10\text{ cm}$ gives $v = +20\text{ cm}$, indicating a real and inverted image at $20\text{ cm}$ on the other side.+1

   Answer: (d)
4. Q4Multiple choice·1 mark
   To obtain a magnified and erect image using a convex lens as a simple microscope, the object must be placed:

   1. (a)At the focus $f$
   2. (b)Within the focal length ($< f$)
   3. (c)Between $f$ and $2f$
   4. (d)At a distance greater than $2f$
   Show answer and step marks

   1. Correct option: (b) Within the focal length ($< f$). A convex lens acts as a simple microscope when the object is placed within its focal length, forming a virtual, erect, and magnified image on the same side as the object.+1

   Answer: (b)
5. Q5Long answer·4 marks
   A convex lens of focal length $15\text{ cm}$ is used to form an image of an object. (a) An object is placed at a distance of $25\text{ cm}$ in front of this lens. Calculate the distance of the image from the lens and state its nature. (b) If the height of the object is $4\text{ cm}$, calculate the height of the image formed.
   Show answer and step marks

   1. Using the lens formula $\frac{1}{f} = \frac{1}{v} - \frac{1}{u}$ with sign convention ($f = +15\text{ cm}$, $u = -25\text{ cm}$).+1
   2. Substituting: $\frac{1}{15} = \frac{1}{v} - \frac{1}{-25} \implies \frac{1}{v} = \frac{1}{15} - \frac{1}{25} = \frac{2}{75}$, so $v = +37.5\text{ cm}$.+0.5
   3. Since $v$ is positive, the image is formed on the other side of the lens: it is real and inverted.+0.5
   4. Using the magnification formula $m = \frac{v}{u}$, substitute $v = +37.5\text{ cm}$ and $u = -25\text{ cm}$ to get $m = -1.5$.+1
   5. Calculating image height $h_2 = m \times h_1 = -1.5 \times 4\text{ cm} = -6\text{ cm}$ (height is $6\text{ cm}$, inverted).+1

   Answer: Image distance = $37.5\text{ cm}$, Nature = Real and inverted, Height of image = $6\text{ cm}$

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