i have a beginner telescope of 50mm and want to see planets it is arefractor with f/12 ratio and two eyepieces of 5mm and 10mm and i want to see mars, Jupiter, Saturn and venus. my max zoom ix 120x and focal length is 600mm
Venus, Mars, Jupiter, and Saturn are visible with your eyes, so a telescope and magnification are not required to see them. Depending on the quality of the telescope, the magnification will show you some details. Venus - you will be able to see the phase. Mars - for several months every 26 months, you will be able to see some dark markings and a polar cap. Jupiter - you will see the cloud bands and 4 of its moons. Saturn - you will see its rings and moon Titan.
Enjoy the view!
The 10 mm ocular should work fine. It will give you 60x of magnification.
The 5 mm ocular is probably a bit too much for this scope. You can definitely try it, but the expectation here is that the image will be a bit too washed out. With 50 mm of aperture you can't really expect to push magnification much above 100x (twice the aperture in mm), and even at around 100x it would work only if it's a high performance instrument. But, like I said, try and see what happens.
What you can see in 50 mm of aperture:
You will see the phases of Venus. It will look like the Moon, just very tiny. This is best done when Venus is at the greatest distance from the Sun, and then it will look like a disk cut in half.
Jupiter - you will definitely see the disk. Seeing the equatorial belts will be more challenging, but it should be doable; it may take a few attempts if it doesn't work on first try. Jupiter is low contrast and it's hard to see its features. Let the image float through the field of view, relax and just take it in; after a few minutes your brain adapts to the view and starts perceiving more details.
Jupiter's moons are definitely visible, like 4 tiny stars that seem to follow the planet around. Sometimes some of them are hidden, so don't be surprised if you can't see them all.
Saturn's ring is clearly visible. You won't see details such as the Cassini division, but the ring itself will be shown across the planet's disk.
The Moon is an excellent target for telescopes of all sizes, and there's a lot to explore there.
Mars will probably not show a lot of details in 50 mm of aperture. Anyway, this planet is only worth observing for a few weeks around each opposition, which happens once every two years (plus a few weeks). At best you will resolve Mars as a tiny brick-red disk, but I doubt you'll see terrain features. But go ahead and give it a try; when the Hellas Basin is full of fog, or frosted over, it's very bright, so in a small aperture it might be perceived as a white speck of light on one side of the disk.
(I'm actually kind of curious myself if this would work. I'll give it a try next time around, since I have a few 50 mm doublets to play with.)
It's important to focus the instrument well before using it. E.g. point it at the Moon and adjust the focus until the image is as sharp as possible. You can also focus on stars - adjust focus until the image of the star is as small as it gets. You can also adjust focus on whatever object you're currently observing.