Question

A ray of light travelling in air enters obliquely into water. This light ray

• A. bends away from the normal
• B. passes through the normal at the surface of separation
• C. bends towards the normal
• D. travels straight without bending

Hint:

Remember the rule: Light bends towards the normal when going from a rarer to a denser medium and bends away from the normal when going from a denser to a rarer medium.

Answer 1

The Correct Option is C

Step 1: Recall the laws of refraction.
When a ray of light travels from one optical medium to another, it bends at the interface between the two media. This phenomenon is called refraction. The extent of bending depends on the refractive indices of the two media and the angle of incidence. Snell's law governs this bending: \[ n_1 \sin \theta_1 = n_2 \sin \theta_2 \] where \( n_1 \) and \( n_2 \) are the refractive indices of the first and second media, respectively, and \( \theta_1 \) and \( \theta_2 \) are the angles of incidence and refraction, respectively, measured with respect to the normal to the surface. 
Step 2: Identify the optical densities of air and water.
Air is a rarer medium (lower optical density, refractive index \( n_{air} \approx 1 \)), and water is a denser medium (higher optical density, refractive index \( n_{water} \approx 1.33 \)). 
Step 3: Apply Snell's law for light travelling from a rarer to a denser medium.
When light travels from a rarer medium (air) to a denser medium (water), \( n_1<_2 \). According to Snell's law: \[ \sin \theta_2 = \frac{n_1}{n_2} \sin \theta_1 \] Since \( \frac{n_1}{n_2}<1 \), we have \( \sin \theta_2<\sin \theta_1 \). For angles between 0 and 90 degrees, a smaller sine value corresponds to a smaller angle. Therefore, \( \theta_2<\theta_1 \). 
Step 4: Interpret the change in angle with respect to the normal.
The angle of refraction \( \theta_2 \) is smaller than the angle of incidence \( \theta_1 \). This means that the refracted ray bends towards the normal. 
Step 5: Consider the case of light incident along the normal.
If the ray of light enters perpendicularly to the surface (along the normal), the angle of incidence \( \theta_1 = 0^\circ \). In this case, \( \sin \theta_1 = 0 \), and from Snell's law, \( \sin \theta_2 = 0 \), which means \( \theta_2 = 0^\circ \). So, there is no bending, and the light passes straight through. However, the question states that the light enters obliquely. Therefore, when a ray of light travels from air to water obliquely, it bends towards the normal.