It is a very fundamental concept that we are able to see something only when light rays fall on it. Try to show what dark and light is to you toddler using following ways:
- Flash a torch light on her palm and show her how the beam of light falls on her palm
- Switch off lights in a room to show what dark is
- Show her the surroundings at night to know that it gets dark in the night
- Show her how morning looks like - bright and sunny
- Show her shadow and try explaining her that there is shadow only when there is some light. This play activity could help.