Teachoo Questions

Class 10
Chapter 10 Class 10 - Light - Reflection and Refraction

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## (ii) List four characteristics of the image (nature, position, size, erect/inverted) formed by the lens in this case

The image formed by the concave lens is

Virtual

Formed 20cm infront of the lens (same side as that of object)

Diminished (smaller than the size object)

Erect

## List four precautions which a student should observe while determining the focal length of a given convex lens by obtaining an image of a distant object on a screen.

The student should take the following precautions

• The lens should be held straight in a vertical position such that its faces are parallel to the screen.
• Clearest  and sharpest  image should be  obtained by adjusting  the position of lens
• At least three  observations should  be taken to get the most accurate result.
• Base  of lens,  screen and  measuring scale  should be in a straight  line.

## If the image formed by a spherical mirror for all positions of the object placed in front of it is always erect and diminished, what type of mirror is it?

Only a convex mirror always forms an erect and diminished image.

## State the laws of refraction of light. Explain the term ‘absolute refractive index of a medium’ and write an expression to relate it with the speed of light in vacuum.

Laws of Refraction

The two laws of refraction of light are

1. Law 1
The Incident Ray,Refracted Ray and Normal all lie in Same plane
1. Law 2
The Ratio of sine of Angle of incidence and Angle of Refraction is constant for a Particular Medium.
This is also known as Snell’s Law.

Absolute Refractive Index of a Medium
The Refractive Index of a Medium with respect to vacuum is called Absolute Refractive Index.
The formula is
Refractive Index of a Medium in Vaccum = Speed of Light in Vaccum/Speed of Light in Medium

## Consider the path of a ray of light passing through a rectangular glass slab for different angles of incidence.

(i) Which one is greater: angle of incidence or angle of refraction?

(ii) What happens to the emergent angle on increasing the incident angle at air-glass interface?

(iii) State the conditions when no bending occurs.

(i) The ray of light enters from air to glass

Hence, it goes from rarer medium to denser medium.

As the light enters from rarer to denser medium, it moves towards the normal.

Hence, the angle of refraction will be less than the angle of incidence.

Angle of incidence will be greater.

(ii) We know that,

Angle of Emergence = Angle of Incidence

As we increase the angle of incidence, the angle of emergence will also keep on increasing.

(ii) There can be two conditions where no bending occurs

1. When ray of light falls perpendicular to the surface
i.e. when ray of light is parallel to the normal
2. When refractive index of both media are equal

## Position of screen = 74.0 cm

1. What is the focal length of the convex lens?
2. Where will the image be formed if he shifts the candle towards the lens at a position of 38 cm?
3. Draw a ray diagram to show the formation of the image in case (ii) as said above?