Reflectors vs. Refractors Revision Notes | A Level AQA Physics

1. Two Ways to Catch Starlight

Every telescope has one fundamental job: collect as much light as possible and bring it to a focus. The question is how you do that — and the answer splits all optical telescopes into two families: refractors, which bend light through lenses, and reflectors, which bounce light off mirrors.

At A-level, you need to go beyond naming these two types. The real exam skill is being able to compare their optical behaviour, explain the aberrations that limit each design, and evaluate which type is better suited to serious astronomical work — and why.

2. How a Refracting Telescope Works

Picture a long tube with a large convex lens at the front — the objective lens — and a smaller convex lens at the eye end — the eyepiece. Parallel rays of light from a distant star enter the tube, strike the objective lens, and are refracted (bent) so they converge to a real image at the focal point. The eyepiece then acts like a magnifying glass, taking that real image and producing a magnified virtual image for the observer.

The key quantity here is the objective lens diameter (the aperture). A larger aperture collects more light, which is essential for seeing faint, distant objects. The ratio of the objective focal length to the eyepiece focal length gives the angular magnification.

The refracting design has an appealing simplicity: the optical path is sealed inside the tube, protecting the optics from dust and air currents. For small, portable instruments used visually — such as those used historically for surveying or by amateur astronomers — refractors are robust and reliable.

3. How a Reflecting Telescope Works

Now replace the objective lens with a large concave primary mirror. Parallel rays from a distant star travel down the open tube, strike the curved mirror surface, and are reflected back up the tube, converging towards a focal point. In the most common design — the Newtonian reflector — a small flat secondary mirror intercepts the converging beam and deflects it to the side of the tube, where the eyepiece sits.

The critical insight is that reflection obeys a single law for all wavelengths: the angle of incidence equals the angle of reflection. This has profound consequences for image quality, which we will return to shortly when we examine aberrations.

4. Chromatic Aberration — The Lens Problem

Here is where refractors run into serious trouble. When white light passes through a glass lens, different wavelengths are refracted by different amounts — this is the same dispersion that creates a rainbow through a prism. Violet light bends more than red light. As a result, each colour comes to focus at a different distance behind the lens.

When you focus the eyepiece on the red image, the violet image is already past focus and appears as a blurred purple halo around the object. When you focus on violet, red smears outward. The result is a coloured fringe — typically purple or blue-red — surrounding every bright star or planet. This defect is called chromatic aberration.

Chromatic aberration worsens with larger aperture because rays at greater distances from the optical axis pass through regions of the lens with greater refracting power, increasing the spread of focal lengths for different wavelengths. This is one of the central reasons why very large refracting telescopes are impractical: the aberration becomes severe and extremely difficult to correct.

Lens designers can partially correct chromatic aberration by combining two lenses made from different types of glass — a converging crown glass element paired with a diverging flint glass element — to form an achromatic doublet. This brings two wavelengths (typically red and blue) to the same focal point, greatly reducing the coloured fringe. However, it cannot eliminate the aberration entirely, and the correction becomes increasingly difficult and expensive as aperture grows.

5. Spherical Aberration — A Problem for Both Designs

Spherical aberration is a different kind of defect, and it can affect both lenses and mirrors. To understand it, picture a concave mirror with a perfectly spherical surface. Rays that strike near the centre of the mirror are reflected to a focal point at a certain distance. Rays that strike near the outer edge of the mirror are reflected to a focal point that is slightly closer to the mirror.

The result is that there is no single sharp focal point — instead, there is a range of focal positions spread along the optical axis. When you place the detector or eyepiece at any one position, the image appears blurred, because rays from different zones of the mirror are not all converging at the same spot. This blurring is spherical aberration.

The same effect occurs in a spherical lens: rays through the outer zones refract more strongly than rays through the centre, again producing a spread of focal points rather than a single sharp focus. Spherical aberration therefore afflicts both refractors and reflectors if their optical surfaces are spherical.

The standard solution for mirrors is to grind the primary mirror to a parabolic rather than spherical shape. A paraboloid focuses all parallel rays — regardless of which zone they strike — to exactly the same focal point, eliminating spherical aberration. This is why professional telescope mirrors are parabolic. The Hubble Space Telescope famously suffered from spherical aberration when launched in 1990 because its primary mirror was ground to the wrong shape by a tiny margin of 2.2 micrometres — a powerful illustration of how sensitive this defect is.

6. Comparing the Merits of Reflectors and Refractors

Now that you understand the optical defects, you are in a position to evaluate the two designs properly. This is the kind of higher-order thinking AQA rewards in extended-response questions — not just listing features, but weighing them against each other with clear reasoning.

The weight issue deserves particular attention. A lens can only be supported at its rim — if you try to support it from behind, you block the light path. As a lens grows larger, it becomes heavier, and the glass begins to deform under gravity, distorting the carefully ground optical surface. The largest refracting telescope ever built, the Yerkes refractor in Wisconsin, has an objective lens of 1.02 metres in diameter. Building anything larger is essentially impractical.

A mirror, by contrast, can be supported across its entire rear surface with a network of actuators. Modern telescopes such as the Very Large Telescope (VLT) in Chile use primary mirrors 8.2 metres in diameter, and the Extremely Large Telescope (ELT) under construction will have a segmented primary mirror 39 metres across. These scales are impossible with lenses.

There is also the matter of light transmission. Glass absorbs some radiation, particularly at ultraviolet and infrared wavelengths. For optical astronomy this is a moderate concern, but for observations across a broader electromagnetic spectrum it becomes significant. A mirror with a thin aluminium or silver coating reflects across a much wider range of wavelengths than any glass lens can transmit.

7. Exam Technique and Mark-Winning Strategies

Questions on this topic tend to fall into two categories: those asking you to explain an aberration (cause and effect), and those asking you to evaluate or compare the two telescope types. Each requires a different approach.

When explaining chromatic aberration, the mark-winning chain is: different wavelengths refracted by different amounts → each colour has a different focal length → coloured fringe around image. When explaining spherical aberration, the chain is: spherical surface geometry → outer rays focus closer than central rays → no single focal point → blurred image. Always state the cause before the effect.