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Light, Shadows and Reflections - Formation of Shadows and Pinhole Camera

Grade 6CBSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Light travels in a straight line. This property is known as the Rectilinear Propagation of Light.

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A shadow is formed when an opaque object comes in the path of light, blocking it. To form a shadow, three things are required: a source of light, an opaque object, and a screen.

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The color of a shadow is always black, regardless of the color of the object used to cast the shadow.

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The size of a shadow changes depending on the distance between the light source, the object, and the screen.

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A Pinhole Camera is a simple device without a lens that works on the principle of rectilinear propagation of light.

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The image formed by a pinhole camera is always inverted (upside down) and real (formed on a screen).

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In a pinhole camera, if the distance between the pinhole and the screen is vv and the distance between the pinhole and the object is uu, the size of the image depends on the ratio of these distances.

📐Formulae

Magnification(m)=Height of Image (hi)Height of Object (ho)\text{Magnification} (m) = \frac{\text{Height of Image } (h_i)}{\text{Height of Object } (h_o)}

Magnification(m)=Distance of image from pinhole (v)Distance of object from pinhole (u)\text{Magnification} (m) = \frac{\text{Distance of image from pinhole } (v)}{\text{Distance of object from pinhole } (u)}

hiho=vu\frac{h_i}{h_o} = \frac{v}{u}

💡Examples

Problem 1:

An object of height 15 cm15\text{ cm} is placed at a distance of 50 cm50\text{ cm} from a pinhole camera. If the screen is 10 cm10\text{ cm} away from the pinhole, what will be the height of the image?

Solution:

Given: ho=15 cmh_o = 15\text{ cm}, u=50 cmu = 50\text{ cm}, v=10 cmv = 10\text{ cm}. Using the formula hiho=vu\frac{h_i}{h_o} = \frac{v}{u}, we get: hi=ho×vu=15×1050=15×15=3 cmh_i = h_o \times \frac{v}{u} = 15 \times \frac{10}{50} = 15 \times \frac{1}{5} = 3\text{ cm}.

Explanation:

By applying the ratio of distances to the ratio of heights, we find the image height is 3 cm3\text{ cm}. The image is smaller than the object.

Problem 2:

Why is the image in a pinhole camera inverted?

Solution:

Because light travels in a straight line (rectilinear propagation).

Explanation:

Light from the top of the object travels in a straight line through the pinhole and hits the bottom of the screen, while light from the bottom of the object hits the top of the screen, resulting in an inverted image.

Problem 3:

What happens to the size of the shadow if an object is moved closer to the light source?

Solution:

The size of the shadow increases.

Explanation:

When the object is closer to the source, it blocks a larger area of light rays, causing the shadow cast on the screen to become larger.