How does a convex lens magnify an image? Does it always magnify? We can draw a ray diagram to show the path of light rays through the lens. Case 1: An Object Far From the Lens: First, let's draw a diagram of a convex lens. This lens has two focal points, one on either side, and at equal distances from, the centre line of the lens. Light rays from an object on one side of the lens pass through the lense and are refracted (bent). In order to see what happens to those rays of light from the object, we will look at an object sitting on the base line, far from the lens. In this first example, the object is located well outside the left focus (actually, more than twice as far from the lens as F1). Look at the ray of light originating at the top of the object and travelling straight towards the lens. It bends as it passes through the glass, and passes through F2. (The ray actually bends in a smooth curve inside the glass; for simplicity, we will assume it turns at some angle just as it hits the centre line). The first principle of refraction in a convex lens is that rays of light hitting the lens parallel to the base line will pass through the focus on the other side. Now let's look at a second ray of light, one that leaves the top of the object and passes through F1. When it hits the lens, it will refract such that it leaves the lens parallel to the base line. The second principle of refraction in a convex lens is that rays of light passing through a focus and then hitting the lens will emerge parallel to the base line. These two rays are enough to identify the location of the image. We know the image must appear to be sitting on the base line. Where the two rays intersect will be the 'top' of the image, corresponding to the top of the object. Notice the result: For an object far from the lens, the image appears upside down and smaller than the object. You can obseve this phenomenon for yourself! In a dimly lit room, turn on a light that has a single bare bulb. Hold a magnifying glass above a piece of white paper and move the paper towards or away from the magnifying glass until you see a tiny image of the light bulb appear on the paper. It will be upside down! Now let's look at the second case, where the object will be closer to the lens, but still farther away than F1. Go on to page 2. Resources Content, HTML, graphics & design by Bill Willis 2023