All questions of Light for Class 7 Exam

Rectilinear Propagation of Light

The statement that is true about rectilinear propagation of light is option 'A': Light travels in a straight line.

Explanation:

What is rectilinear propagation of light?
Rectilinear propagation of light refers to the phenomenon where light travels in a straight line through a homogeneous medium. It means that light travels in a straight path unless it is interrupted or influenced by other factors.

Reasons for rectilinear propagation of light:
There are several reasons why light propagates in a straight line:

1. Homogeneous medium: Light travels in a straight line through a homogeneous medium, which means that the properties of the medium do not change significantly. In such a medium, light encounters minimal obstacles or disturbances that could cause it to deviate from a straight path.

2. Waves and particles: Light exhibits properties of both waves and particles. When considered as waves, light travels in straight lines due to the principle of superposition, which states that when two waves meet, the sum of their amplitudes determines the resulting wave. This principle allows the light waves to propagate in a straight line.

3. Law of reflection: When light encounters a smooth and polished surface, it follows the law of reflection, which states that the angle of incidence is equal to the angle of reflection. This phenomenon ensures that light bounces off the surface in a predictable manner, maintaining a straight path.

Common misconceptions:
It is important to clarify common misconceptions related to rectilinear propagation of light:

1. Light can travel in a curved path: While it is true that light can change direction when passing through different mediums or encountering obstacles, it still follows a straight path within each medium or between obstacles. The overall path may appear curved due to multiple straight segments.

2. Light can bend around corners: This statement is not accurate as light cannot bend around corners. However, it can undergo phenomena like diffraction and refraction, which may cause it to change direction when passing through openings or encountering a change in medium.

3. Light can only travel in a zigzag path: This statement is incorrect as light does not travel in a zigzag path. Zigzag motion may occur when light undergoes multiple reflections or refractions, but each individual segment of light travels in a straight line.

In conclusion, the true statement about rectilinear propagation of light is that light travels in a straight line. This behavior is a result of the properties of light waves and particles, the principle of superposition, and the laws of reflection.

Reflection of Light:
Reflection of light is the bouncing back of light when it strikes a surface. It is an important phenomenon that allows us to see objects around us. The reflection of light follows certain rules and can occur on different types of surfaces.

Explanation:
1. Reflection on Smooth Surfaces:
- Reflection can occur on smooth surfaces, but it is not limited to them.
- When light strikes a smooth surface like a mirror or still water, it undergoes regular reflection, where the incident angle is equal to the reflected angle.
- The smoothness of the surface allows the light waves to bounce back in a well-organized manner, resulting in a clear and sharp reflection.

2. Reflection on Any Surface:
- Reflection can occur on any surface, whether it is smooth, rough, curved, or flat.
- When light strikes a rough surface, such as a wall or a piece of paper, it undergoes diffused reflection, where the incident angle is not equal to the reflected angle.
- The roughness of the surface causes the light waves to scatter in different directions, resulting in a hazy or blurred reflection.

3. Reflection on Flat Surfaces:
- Reflection is not limited to flat surfaces.
- While flat surfaces provide a clear and regular reflection, other surfaces can also reflect light.
- Curved surfaces like a spoon, a convex mirror, or a concave mirror can reflect light as well. The reflection on curved surfaces may cause the light to converge or diverge, resulting in different optical effects.

4. Reflection on Curved Surfaces:
- Reflection can occur on curved surfaces, but it is not exclusive to them.
- Curved surfaces can exhibit interesting reflection properties, such as focusing or enlarging the reflected image.
- Mirrors with different curvatures are commonly used to create various optical instruments like telescopes, microscopes, and magnifying glasses.

Conclusion:
In conclusion, the statement "Reflection can occur on any surface" is true. Reflection is not limited to smooth surfaces, flat surfaces, or curved surfaces. It can occur on a wide range of surfaces, including smooth, rough, flat, and curved surfaces. Understanding reflection is important for various applications in daily life and science.