When the refractive index increases, light slows down in the medium.

Light has a funny habit: it changes speed when it passes from one material into another. If you’ve ever watched a straw look bent in a glass of water, you’ve seen a tiny, everyday version of that effect. But today we’re zooming in on a precise question: what happens to the speed of light when the refractive index of a medium goes up?

The straight answer is simple and tidy: when the refractive index n of a medium increases, the speed of light v inside that medium decreases. So the correct option for the multiple-choice question you’re likely to see is B: light speed decreases in the medium.

Let me explain why this is the case, and then we’ll connect it to real-world intuition that helps the idea stick.

Let’s start with a clean formula

• The speed of light in a vacuum is a constant, c, about 3 × 10^8 meters per second.

• In any medium, light doesn’t rush along at c. It travels at a slower speed v, and that speed is related to the refractive index n of the medium by the simple relation:

v = c / n

• The refractive index n is a dimensionless number that tells us how much the medium resists light’s progress compared with vacuum.

What does “increasing n” actually mean?

Think of n as a measure of optical density. If the medium has more material with which light can interact, light has more opportunities to couple with the medium’s electric charges and polarized atoms as it moves. Each interaction takes a tiny moment, and those moments add up. The light doesn’t lose its energy or frequency in the usual sense, but its effective pace through the substance slows.

Two key points to keep in mind:

• Frequency stays the same at an interface. When light passes from one medium to another, its frequency f is conserved, but its wavelength changes to accommodate the new speed: λ = v / f.

• The reduction in speed is exactly what the refractive index predicts. A larger n means a smaller v, because you’re dividing by a bigger number in v = c/n.

A quick mental model

A useful way to picture this is to imagine light as a beam of couriers running through a crowded street. In air, the street is wide and empty, so they move fast (v ≈ c). When the same couriers enter a denser street with more pedestrians, they spend more time weaving through the crowd. Each interaction slows them down a bit, so the overall pace drops. In physics terms, the medium’s optical properties introduce a lag in how quickly the light’s wavefront propagates, which shows up as a smaller v.

Concrete numbers you can anchor to

• In air (almost like vacuum for many purposes), n is very close to 1, so v is near c.

• Water has n ≈ 1.33, so v ≈ c / 1.33 ≈ 2.25 × 10^8 m/s.

• Window glass is around n ≈ 1.5, so v ≈ c / 1.5 ≈ 2.0 × 10^8 m/s.

• Diamond is much denser optically, with n ≈ 2.4, so v ≈ c / 2.4 ≈ 1.25 × 10^8 m/s.

That’s a big spread, and it helps explain why light bends at interfaces. When light hits a new medium, its speed changes, and because the wavefronts have to reorient themselves to the new pace, the direction shifts. This bending is refraction in action, and it’s all tied to how v and n relate.

What about real-world implications?

• Fiber optics: Signals travel through glass fibers where light slows down compared with air, but it’s the total internal reflection that keeps the light wagging along the core. The slower speed does mean a delay, but the advantage is high-bandwidth, long-distance communication.

• Lenses and imaging: A higher n lets you bend light more effectively with compact shapes. That’s why glass, plastics, and liquids with different n are chosen to correct vision or to form images in cameras and telescopes.

• Medical and scientific instruments: The delay and wavelength change in different media affect how we design devices like endoscopes, microscopes, and spectrometers. Small shifts in v and λ can matter a lot when you’re trying to measure something precise.

Common questions (and quick clarifications)

• Does the light lose energy when n increases? Not in the usual sense. The energy carried by a photon is hf, which stays the same when light travels from one part of a medium to another. What changes is how quickly the wavefront travels and how the wavelength fits inside that medium.

• Is the speed always c/n? For the most part, yes, inside a uniform, non-absorbing medium you can use v = c/n as a good description of the phase velocity. In some dispersive media, the group velocity can behave a bit differently, especially across different frequencies, but the basic relation gives a solid starting point.

• Can the speed ever exceed c in a medium? No. The universal speed limit, c, is not surpassed by light once it’s inside any material. The speed slows down to c/n, and although the front of a pulse cannot exceed c, there are tricky dispersion effects that can make certain components appear to travel differently—without breaking that limit.

A few mindful tangents that keep the idea grounded

• Wavelength changes with v. Since frequency stays fixed at the boundary, the wavelength in the medium gets shorter as n grows. This is why the same beam can behave so differently from one medium to another, and it’s also part of why lenses work the way they do.

• Dispersion matters. In many real materials, n depends on wavelength. That means different colors (frequencies) slow down by different amounts. It’s how rainbows form when sunlight passes through a prism—the colors spread out because they travel at slightly different speeds in the glass.

• Not all media slow light the same way. Some substances have an n just a hair above 1; others are much denser optically. The degree to which v shrinks tells you something about how the material interacts with electromagnetic waves.

A friendly takeaway

If you remember one thing, let it be this: increasing the refractive index of a medium makes light slow down inside that medium. The speed is simply the vacuum speed divided by the medium’s refractive index, v = c/n. That crisp relation condenses a lot of physics into a handy rule you can use again and again, whether you’re predicting the path of a laser through glass, thinking about how a camera lens focuses light, or simply curious about how light behaves when it switches air for water.

If the topic sparked curiosity, you’re not alone. Light is full of little surprises, and understanding this basic relation gives you a solid stepping-stone to the bigger ideas—like how speed interacts with energy, how wavelength shifts color, and how devices we rely on every day are designed around light’s motion through different materials.

And that’s the neat part: a single, elegant equation opens up a world of phenomena. The next time you see a straw appearing bent in a glass, you’ll have a ready-made explanation in your back pocket—the refractive index is higher there, and light feels a little more “slowed down” as it makes its way through. The pace changes, but the wonder stays the same.