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Light, Shadows and Reflections - Mirrors and Rectilinear Propagation of Light

Grade 6CBSE

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

🔑Concepts

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Rectilinear Propagation of Light: This is the property of light traveling in straight lines. It is the reason why we cannot see around corners and why shadows are formed.

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Luminous and Non-luminous Objects: Objects like the Sun that give out or emit light of their own are called luminous objects. Objects like the Moon that do not emit light but are visible because they reflect light from a source are non-luminous.

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Transparent, Translucent, and Opaque Objects: Transparent materials allow light to pass through completely (100%100\% transmission). Translucent materials allow light to pass partially. Opaque materials do not allow any light to pass through, resulting in shadows.

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Shadow Formation: A shadow is formed when an opaque object blocks the path of light. A shadow requires a source of light, an opaque object, and a screen to be visible.

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Pinhole Camera: A simple camera without a lens that uses a tiny aperture (the pinhole) to project an inverted image of an object based on the rectilinear propagation of light.

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Mirrors and Reflection: A mirror is a polished surface that reflects light. Reflection is the phenomenon where light bounces off a surface, changing its direction.

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Lateral Inversion: The phenomenon in a plane mirror where the left side of the object appears to be the right side of the image and vice-versa.

📐Formulae

Law of Reflection: ∠i=∠r\text{Law of Reflection: } \angle i = \angle r

Magnification in Pinhole Camera (m)=Height of image (hi)Height of object (ho)\text{Magnification in Pinhole Camera (m)} = \frac{\text{Height of image }(h_i)}{\text{Height of object }(h_o)}

hiho=vu\frac{h_i}{h_o} = \frac{v}{u} (where vv is distance of screen from pinhole and uu is distance of object from pinhole)

💡Examples

Problem 1:

Explain why the image formed by a pinhole camera is inverted using the principle of light.

Solution:

The image is inverted because light travels in straight lines (Rectilinear Propagation). Light from the top of the object travels in a straight line through the pinhole to reach the bottom of the screen, and light from the bottom travels to the top of the screen.

Explanation:

Since light cannot bend to reach the corresponding height on the screen, the paths of light rays from the top and bottom of the object cross at the pinhole, resulting in an upside-down (inverted) image.

Problem 2:

If the angle of incidence of a light ray on a plane mirror is 35∘35^\circ, what is the angle of reflection?

Solution:

∠r=35∘\angle r = 35^\circ

Explanation:

According to the laws of reflection, the angle of incidence (∠i\angle i) is always equal to the angle of reflection (∠r\angle r). Therefore, if ∠i=35∘\angle i = 35^\circ, then ∠r\angle r must also be 35∘35^\circ.

Problem 3:

A candle of height 10 cm10 \text{ cm} is placed in front of a pinhole camera. If the magnification is 0.50.5, what is the height of the image formed?

Solution:

hi=m×ho=0.5×10 cm=5 cmh_i = m \times h_o = 0.5 \times 10 \text{ cm} = 5 \text{ cm}

Explanation:

Using the magnification formula m=hihom = \frac{h_i}{h_o}, we multiply the magnification by the object height to find the image height. The image will be 5 cm5 \text{ cm} tall and inverted.