Light changes direction
When light reaches a surface, some of it may be reflected. Reflection is the return of light into the medium it came from. A plane mirror provides a familiar example.
The angle of incidence equals the angle of reflection. Both angles are measured from the normal, an imaginary line perpendicular to the surface at the point where the ray meets it. The incident ray, reflected ray, and normal lie in the same plane.
Measure angles from the normal, not from the surface.
Why a pencil looks bent in water
Light travels at different speeds in different transparent media. When it crosses an interface at an angle, its direction usually changes. This is refraction.
Light travelling from air into glass bends towards the normal because its speed decreases. Light travelling from glass into air bends away from the normal. At normal incidence, its speed changes but its direction does not.
The apparent bend in a pencil partly immersed in water is caused by refraction at the water-air surface.
Lenses bring rays together or spread them apart
A convex lens is thicker at the centre. In air, it can bring parallel rays towards a principal focus. A concave lens is thinner at the centre and makes parallel rays diverge.
Ray diagrams help locate images. Draw the principal axis, the lens, and its focal points first. Then trace two useful rays from the top of the object. Where the rays, or their backward extensions, meet determines the image position.
A quick thought experiment
Imagine a ray striking a plane mirror at an angle of 30° to the normal. Its reflection angle is also 30°. The angle between the incident and reflected rays is 60°.
Now let the ray enter water at an angle. Its speed decreases and it bends towards the normal. What if it enters directly along the normal? Its speed changes, but it continues straight.