### Correct Option: A. Undeviated and undisplaced at a lower speed #### Detailed Explanation: 1. **Understanding Normal Incidence**: - When a ray of light strikes a surface at normal incidence, it means that the light hits the surface perpendicularly (at a 90-degree angle). In this case, the angle of incidence is 0 degrees. 2. **Behavior of Light at the Interface**: - When light travels from one medium to another (in this case, from air to glass), its speed changes due to the different optical densities of the two media. Air has a lower refractive index compared to glass. - The refractive index (n) of air is approximately 1.0, while that of glass is typically around 1.5. The speed of light in a medium can be calculated using the formula: \[ v = \frac{c}{n} \] where \( c \) is the speed of light in a vacuum (approximately \( 3 \times 10^8 \) m/s). 3. **Speed of Light in Glass**: - Using the refractive index of glass: \[ v_{\text{glass}} = \frac{3 \times 10^8 \text{ m/s}}{1.5} = 2 \times 10^8 \text{ m/s} \] - This shows that light travels slower in glass than in air. 4. **Refraction at Normal Incidence**: - According to Snell's Law, which states that \( n_1 \sin(\theta_1) = n_2 \sin(\theta_2) \), when light strikes at normal incidence (\(\theta_1 = 0\)), the equation simplifies to: \[ n_1 \cdot 0 = n_2 \cdot \sin(\theta_2) \] This means that \(\theta_2\) is also 0 degrees, indicating that the light continues straight through the glass without bending (deviation). 5. **Displacement of the Ray**: - Although the light does not bend, it does travel through the glass slab, which means it will exit the slab at the same angle it entered. However, the path of the light ray is effectively "displaced" because it travels through the thickness of the glass slab. This displacement is not a deviation in direction but rather a shift in position. 6. **Conclusion**: - Therefore, the light ray is **undeviated** (it continues in the same direction) and **undisplaced** in terms of its angle, but it travels at a **lower speed** while in the glass. Hence, the correct answer is **A**. #### Why Other Options Are Incorrect: - **Option B: Deviated and undisplaced at a lower speed**: - This option incorrectly states that the light is deviated. Since the light strikes at normal incidence, it does not deviate. - **Option C: Deviated and displaced at a lower speed**: - Similar to option B, this option incorrectly suggests that the light is deviated. The light does not change direction at normal incidence. - **Option D: Undeviated and displaced at a faster speed**: - This option is incorrect because, while the light is undeviated, it does not travel at a faster speed in glass; it travels at a lower speed due to the higher refractive index of glass compared to air. ### Revision Summary: - Light striking a surface at normal incidence does not deviate; it continues straight through. - The speed of light decreases when entering a denser medium like glass. - The light ray is undeviated and travels at a lower speed in glass. - Displacement refers to the shift in position, not a change in direction, when light passes through a medium.