Class 10 > Unit # 10: General Waves Properties > Reflection, Refraction & Diffraction of Waves

Q.2 Define the Three Properties of Waves: Reflection, Refraction, and Diffraction

Video Lecture

i) Reflection of Waves

Definition: Bouncing back of waves into the same medium by striking the surface of another medium is called reflection of waves.

When a vertical, straight barrier is placed in the path of incoming water waves inside a ripple tank, the incident waves strike the surface and bounce back into the original medium. These reflected waves strictly obey the Law of Reflection:

\[ \theta_i = \theta_r \]

Where \(\theta_i\) represents the angle of incidence measured relative to the normal line, and \(\theta_r\) represents the angle of reflection.

Figure 1: Reflection of plane water waves at a flat obstacle obeying the Law of Reflection.

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ii) Refraction of Waves

Definition: When waves enter from one medium into another medium, the resulting change in their direction and speed is called refraction of waves.

When a flat glass block is submerged in a ripple tank, the depth of the water becomes shallower over the block. As incoming plane waves transition from deep water to shallow water:

• The wavelength (\(\lambda\)) shortens (\(\lambda_{\text{shallow}} < \lambda_{\text{deep}}\)).
• The wave speed (\(v\)) decreases (\(v_{\text{shallow}} < v_{\text{deep}}\)).
• The frequency (\(f\)) remains unchanged because it is determined solely by the source vibrator.

This relation is expressed through the fundamental wave equation:

\[ v = f \cdot \lambda \implies \frac{v_{\text{deep}}}{\lambda_{\text{deep}}} = \frac{v_{\text{shallow}}}{\lambda_{\text{shallow}}} = f \]

Figure 2: Refraction of water waves demonstrating a change in wavelength and direction across depth boundaries.

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iii) Diffraction of Waves

Definition: The bending of waves around an obstacle or the spreading of waves near an obstacle or gap is called diffraction of waves.

When straight water waves strike a barrier with a small gap in a ripple tank, they bend around the edges and spread outward. Diffraction becomes significant when the width of the gap (\(d\)) is approximately equal to the wavelength (\(\lambda\)) of the incident wave:

\[ d \approx \lambda \]

When the gap width is small (\(d \approx \lambda\)), the waves emerging from the gap become almost circular, acting as if they originate from a single point source inside the slit. If the gap is significantly wider than the wavelength (\(d \gg \lambda\)), very little bending occurs and diffraction is negligible.

Figure 3: Diffraction of plane water waves passing through a slit comparable in size to their wavelength.