Download App

Light – Reflection and Refraction — Science STD 10 — Question

CBSE BoardEnglish MediumSTD 10ScienceLight – Reflection and Refraction3 Marks
Question
When an object is placed 20cm from a concave mirror, a real image magnified three times is formed. Find:
  1. The focal length of the mirror.
  2. Where must the object be placed to give a virtual image three times the height of the object?
✓

Answer

Given:u = -20cm, m = -3cm, for the real image
  1. We know that
$\text{m}=-\frac{\text{m}}{\text{u}}$
$\therefore \text{m}=-3=-\frac{\text{v}}{(-20)}$
$\Rightarrow\text{v}= -60\text{cm}$
We have
$\frac{1}{\text{v}}+\frac{1}{\text{u}}=\frac{1}{\text{f}}$
$\Rightarrow \frac{1}{(-60)}+\frac{1}{(-20)}=\frac{1}{\text{f}}$
$\frac{1}{\text{f}}=-\frac{1}{60}-\frac{1}{20}=\frac{-1-3}{60}=\frac{1}{15}$
$\text{f}=-15\text{cm}$
  1. For virtual image m = 3 , and f = 15cm
$\text{m}=-\frac{\text{v}}{\text{u}}$
$\therefore \text{m}=-3=-\frac{\text{v}}{(-20)}\Rightarrow \text{v}=-3\text{u}$
We have
$\frac{1}{\text{v}}+\frac{1}{\text{u}}=\frac{1}{\text{f}}$
$\Rightarrow \frac{1}{\text{(3u)}}+\frac{1}{\text{u}}=\frac{1}{(-15)}$
$\Rightarrow \frac{-1+3}{\text{3u}}=-\frac{1}{15}$
$\Rightarrow \text{u}=-\frac{2\times15}{3}=-10\text{cm}$
So object should be placed 10cm from the concave mirror.

Need a full question paper?

Generate a complete, print-ready paper with questions like this in minutes — across 16+ boards, with answer keys.

Start Generating Free

Explore more

More "[3 Mark Questions]" from Light – Reflection and Refraction→Light – Reflection and Refraction — all question types→Science STD 10 question papers→CBSE Board STD 10 question papers→CBSE Board question paper generator→

Similar questions

Usually, three insulated wires of different colours are used in an electrical appliance. Identify the three colours.
→
Explain how, geothermal energy is used to generate electricity.
→
Here are some results of solutions tested with universal indicator paper:
  1. Sulphuric acid: Red
  2. Metal polish: Dark blue
  3. Washing-up liquid:
  4. Yellow Milk of magnesia:
  5. Light blue Oven cleaner:
  6. Purple Car battery acid: Pink
Arrange the solutions in order of their increasing pH values (starting with the one with the lowest pH).
→
$i.$ Distinguish between ionic and covalent compounds under the following properties:
$a.$ Strength of forces between constituent elements
$b.$ Solubility of compounds in water
$c.$ Electrical conduction in substances
$ii.$ Explain how the following metals are obtained from their compounds by the reduction process:
$a.$ Metal $M$ which is in the middle of the reactivity series.
$b.$ Metal $N$ which is high up in the reactivity series. Give one example of each type.
→
A dialysis machine contains long tubes coiled in a tank containing dialysing solution:
  1. Of what substance are the tubes made?
  2. What does the dialysing solution contain?
  3. Name the main waste which passes into the dialysing solution.
→
What is nuclear fission? Explain with an example. Write the equation of the nuclear reaction involved.
→
An object of size 7.0cm is placed at a distance of 27cm in front of concave mirror of focal length 18cm. At what distance from the mirror should a screen be placed so that a sharply focused image can be obtained? Find the size and the nature of the image.
→
Explain with the help of a labelled ray diagram, why a pencil partly immersed in water appears to be bent at the water surface. State whether the bending of pencil will increase or decrease if water is replaced by another liquid which is optically more dense than water. Give reason for your answer.
→
What is lymph? State two major functions of lymph.
→
How does a solenoid behave like a magnet? Can you determine the north and south poles of a current–carrying solenoid with the help of a bar magnet? Explain.
→