Learn Refraction of light through a glass slab

Have you ever noticed how a pencil looks bent when placed in a glass of water? This fascinating optical effect is caused by refraction, which is the bending of light as it travels from one transparent medium to another. In your CBSE Class 10 Science syllabus (Light – Reflection and Refraction), understanding the refraction of light through a rectangular glass slab is a critical foundation for optics. When light passes from a medium like air into a different medium like glass, its speed changes. This change in speed forces the light ray to alter its straight-line path, resulting in a visible "bend."

The core logic of this phenomenon involves two distinct boundaries. When the incident ray travels from a rarer medium (air) to a denser medium (glass), it slows down and bends towards the normal, forming the refracted ray and the angle of refraction (r). Conversely, when this light exits the glass slab and returns to the air, it speeds up and bends away from the normal, becoming the emergent ray. Because the top and bottom surfaces of a rectangular glass slab are perfectly parallel, the bending effect at the first interface is exactly reversed at the second. As a mathematical and logical result, the angle of incidence (∠i) is always equal to the angle of emergence (∠e).

In the ray diagram above, trace the path of the red light ray step-by-step. You can clearly see it bending towards the normal line (N) at the top interface as it enters the denser glass, and then bending away from the normal line (M) at the bottom interface as it exits into the air. Notice the dashed line representing the extrapolated original path of the incident ray. The emergent ray is perfectly parallel to this original path but has shifted slightly to the side. This perpendicular distance between the original path and the actual emergent ray is known as lateral displacement. Mastering this exact diagram is highly practical, as you will frequently be asked to draw and label it during your board exams or practical laboratory assessments to prove that the angle of incidence equals the angle of emergence.

Grasping the precise geometry of light rays and optics can sometimes be challenging when you are independently preparing for your Class 10 board exams. If you need help mastering ray diagrams, refraction rules, or any other critical Science topics, UrbanPro is here to help. You can easily find highly experienced, verified Class 10 Science tutors on the UrbanPro platform who offer both personalized online sessions and local offline tuition tailored to your learning pace. Book a session today to build a strong foundation in Physics and ace your upcoming exams with confidence!